feat(gitcommit): skip auto-generation on amend commits

- add skip_auto_generate_on_amend config option
- improve amend detection by checking COMMIT_EDITMSG content
- prevent auto-generation if editing an existing commit message
- update buffer, config, and types to support new option

Author: jinzhongjia

lua/codecompanion/_extensions/gitcommit/buffer.lua

@@ -5,12 +5,12 @@ local Generator = require("codecompanion._extensions.gitcommit.generator")
 ---@class CodeCompanion.GitCommit.Buffer
 local Buffer = {}
 
----@type CodeCompanion.GitCommit.ExtensionOpts.Buffer Default configuration
 local default_config = {
   enabled = true,
   keymap = "<leader>gc",
   auto_generate = false,
   auto_generate_delay = 100, -- Default delay in ms
+  skip_auto_generate_on_amend = true, -- Skip auto-generation during git commit --amend
 }
 
 ---@type table Current configuration
@@ -32,19 +32,20 @@ function Buffer.setup(opts)
       Buffer._setup_gitcommit_keymap(event.buf)
 
       if config.auto_generate then
-        -- This autocommand will trigger once when the user enters the gitcommit window.
-        -- This avoids race conditions with plugins like neogit that manage multiple windows.
+         -- Auto-generation autocommand triggers once when entering gitcommit window
+         -- This avoids race conditions with plugins like neogit that manage multiple windows
+         -- Skip auto-generation during git amend to preserve user's intent to modify existing commit
         vim.api.nvim_create_autocmd("WinEnter", {
           buffer = event.buf,
           once = true,
           callback = function(args)
-            -- Defer the execution to ensure other plugins (like neogit) have finished their UI setup.
+             -- Defer execution to ensure other plugins (like neogit) have finished UI setup
             vim.defer_fn(function()
               if not vim.api.nvim_buf_is_valid(args.buf) then
                 return
               end
 
-              -- Check if buffer already has a commit message (e.g., during amend)
+               -- Check if buffer already has a commit message
               local lines = vim.api.nvim_buf_get_lines(args.buf, 0, -1, false)
               local has_message = false
               for _, line in ipairs(lines) do
@@ -54,9 +55,13 @@ function Buffer.setup(opts)
                 end
               end
 
-              if not has_message then
-                Buffer._generate_and_insert_commit_message(args.buf)
-              end
+               -- Skip auto-generation in the following cases:
+               -- 1. Buffer already has a commit message
+               -- 2. We are in a git amend operation (user may want to keep existing message)
+               local should_skip_amend = config.skip_auto_generate_on_amend and Git.is_amending()
+               if not has_message and not should_skip_amend then
+                 Buffer._generate_and_insert_commit_message(args.buf)
+               end
             end, config.auto_generate_delay)
           end,
           desc = "Auto-generate GitCommit message",

lua/codecompanion/_extensions/gitcommit/config.lua

@@ -23,6 +23,7 @@ M.default_opts = {
     keymap = "<leader>gc",
     auto_generate = true,
     auto_generate_delay = 200,
+    skip_auto_generate_on_amend = true, -- Skip auto-generation during git commit --amend
   },
   add_slash_command = true,
   add_git_tool = true,

lua/codecompanion/_extensions/gitcommit/git.lua

@@ -133,22 +133,57 @@ function Git.is_amending()
       return false
     end
 
-    -- Check if COMMIT_EDITMSG exists and we're in a rebase/merge state
-    local git_dir = vim.fn.system("git rev-parse --git-dir"):gsub("\n", "")
-    if vim.v.shell_error ~= 0 then
-      return false
-    end
-
-    -- Check for COMMIT_EDITMSG file which indicates we're editing a commit
-    local commit_editmsg = git_dir .. "/COMMIT_EDITMSG"
-    local stat = vim.uv.fs_stat(commit_editmsg)
-    if not stat then
-      return false
-    end
-
-    -- Additional check: see if we have HEAD commit (not initial commit)
-    vim.fn.system("git rev-parse --verify HEAD")
-    return vim.v.shell_error == 0
+     -- Check if we're in an amend scenario by examining the COMMIT_EDITMSG content
+     -- During amend, git pre-populates COMMIT_EDITMSG with the previous commit message
+     local git_dir = vim.trim(vim.fn.system("git rev-parse --git-dir"))
+     if vim.v.shell_error ~= 0 then
+       return false
+     end
+ 
+     -- Use platform-appropriate path separator
+     local path_sep = package.config:sub(1, 1)
+     local commit_editmsg = git_dir .. path_sep .. "COMMIT_EDITMSG"
+     local stat = vim.uv.fs_stat(commit_editmsg)
+     if not stat then
+       return false
+     end
+ 
+     -- Additional check: verify HEAD commit exists (not initial commit)
+     local redirect = (vim.uv.os_uname().sysname == "Windows_NT") and " 2>nul" or " 2>/dev/null"
+     vim.fn.system("git rev-parse --verify HEAD" .. redirect)
+     if vim.v.shell_error ~= 0 then
+       return false
+     end
+ 
+     -- Read COMMIT_EDITMSG content and check if it contains a previous commit message
+     -- During amend, git pre-populates this file with the previous commit message
+     local fd = vim.uv.fs_open(commit_editmsg, "r", 438)
+     if not fd then
+       return false
+     end
+ 
+     local content = vim.uv.fs_read(fd, stat.size, 0)
+     vim.uv.fs_close(fd)
+     
+     if not content then
+       return false
+     end
+ 
+     -- Check if there's non-comment content in COMMIT_EDITMSG
+     -- During amend, this indicates we're editing an existing commit
+     local lines = vim.split(content, "\n")
+     local has_existing_message = false
+     for _, line in ipairs(lines) do
+       local trimmed = vim.trim(line)
+       -- Skip empty lines and comment lines (starting with #)
+       if trimmed ~= "" and not trimmed:match("^#") then
+         has_existing_message = true
+         break
+       end
+     end
+ 
+     -- If COMMIT_EDITMSG has existing content and HEAD exists, likely an amend
+     return has_existing_message
   end)
 
   return ok and result or false

lua/codecompanion/_extensions/gitcommit/types.lua

@@ -8,9 +8,9 @@
 ---@field enabled boolean Enable buffer-specific keymap for git commit
 ---@field keymap string Keymap for generating commit message in git commit buffer
 ---@field auto_generate? boolean Automatically generate commit message on entering gitcommit buffer
----@field auto_generate_delay? number Delay in ms before auto-generating to avoid race conditions
 
----@field add_slash_command? boolean Add /gitcommit slash command to chat buffer
+ ---@field skip_auto_generate_on_amend? boolean Skip auto-generation during git commit --amend operations
+ 
 ---@field buffer? CodeCompanion.GitCommit.ExtensionOpts.Buffer Enable buffer-specific keymap for git commit
 ---@field adapter? string? The adapter to use for generation
 ---@field model? string? The model of the adapter to use for generation