'git interpret-trailers' is tripped by comment characters other than '#'

Hi.

When I tried to add ChangeId tag for Gerrit Code Review, I noticed 'git interpret-trailers' went wrong if a comment character other than '#' is used.

Quick Test Code ---------------

cat <<EOF | git -c trailer.ifexists=doNothing interpret-trailers \
      --trailer "Change-Id: new tag"
subject: this is commit subject

Blah Blah

Change-Id: old tag
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

; This is a comment line with non-default char

EOF

[result]

subject: this is commit subject

Blah Blah

Change-Id: old tag
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

; This is a comment line with non-default char

Change-Id: new tag

The same trailer 'Change-Id' was appended, ignoring trailer.ifexists=doNothing

Details -------

For Gerrit Code Review, we add "Change-Id" tag, which will identify the patch though the review process.

For details, you can refer: https://www.gerritcodereview.com/cmd-hook-commit-msg.html

Gerrit provides a shell script "commit-msg", which is hooked from 'git commit'.

You can see its implementation here: https://github.com/GerritCodeReview/gerrit/blob/v3.0.0/resources/com/google/gerrit/server/tools/root/hooks/commit-msg

It is implemented by using 'git interpret-trailers'.

I prefer ';' to '#" for commit comment lines.

So, I add the following:

[core]
        commentChar = ";"

With core.commentChar is set, 'git interpret-trailers' is so confused that accumulate the same tag for every 'git commit --amend'.

I guess this should be fixed on the Git side.

Perhaps, 'git interpret-trailers' should be changed to recognize core.commentChar ?