Allow empty-valued commit headers in more places

I just fixed a problem where a `gpgsig ` header line in a historical
commit in the git.git repository itself was mistakenly causing a parse
error. The culprit was, of course, that the header value is empty, and
the existing code expected the value _not_ to be empty.

Handle similar issues in other locations, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

gix-object/src/commit/decode.rs

@@ -44,7 +44,7 @@ pub fn commit<'a, E: ParserError<&'a [u8]> + AddContext<&'a [u8], StrContext>>(
             .context(StrContext::Expected("author <signature>".into())),
         (|i: &mut _| parse::header_field(i, b"committer", parse::signature))
             .context(StrContext::Expected("committer <signature>".into())),
-        opt(|i: &mut _| parse::header_field(i, b"encoding", take_till(1.., NL)))
+        opt(|i: &mut _| parse::header_field(i, b"encoding", take_till(0.., NL)))
             .context(StrContext::Expected("encoding <encoding>".into())),
         repeat(
             0..,

gix-object/src/commit/ref_iter.rs

@@ -214,7 +214,7 @@ impl<'a> CommitRefIter<'a> {
                 }
             }
             Encoding => {
-                let encoding = opt(|i: &mut _| parse::header_field(i, b"encoding", take_till(1.., NL)))
+                let encoding = opt(|i: &mut _| parse::header_field(i, b"encoding", take_till(0.., NL)))
                     .context(StrContext::Expected("encoding <encoding>".into()))
                     .parse_next(input)?;
                 *state = State::ExtraHeaders;
@@ -227,7 +227,7 @@ impl<'a> CommitRefIter<'a> {
                 let extra_header = opt(alt((
                     |i: &mut _| parse::any_header_field_multi_line(i).map(|(k, o)| (k.as_bstr(), Cow::Owned(o))),
                     |i: &mut _| {
-                        parse::any_header_field(i, take_till(1.., NL))
+                        parse::any_header_field(i, take_till(0.., NL))
                             .map(|(k, o)| (k.as_bstr(), Cow::Borrowed(o.as_bstr())))
                     },
                 )))

gix-object/src/parse.rs

@@ -18,7 +18,7 @@ pub(crate) fn any_header_field_multi_line<'a, E: ParserError<&'a [u8]> + AddCont
     (
         terminated(take_till(1.., SPACE_OR_NL), SPACE),
         (
-            take_till(1.., NL),
+            take_till(0.., NL),
             NL,
             repeat(1.., terminated((SPACE, take_until(0.., NL)), NL)).map(|()| ()),
         )