commit: avoid scanning trailing comments when 'core.commentChar' is "auto"

commit: avoid scanning trailing comments when 'core.commentChar' is "auto"

When core.commentChar is set to "auto", Git selects a comment character
by scanning the commit message contents and avoiding any character
already present in the message.

If the message still contains old conflict comments (starting with a
comment character), Git assumes that character is in use and chooses a
different one. As a result, those existing comment lines are no longer
recognized as comments and end up being included in the final commit
message.

To avoid this, skip scanning the trailing comment block when selecting
the comment character. This allows Git to safely reuse the original
character when appropriate, keeping the commit message clean and free of
leftover conflict information.

Background:

The "auto" value for core.commentchar was introduced in the commit
84c9dc2c5a (commit: allow core.commentChar=auto for character auto
selection, 2014-05-17) but did not exhibit this issue at that time.

The bug was introduced in commit a6c2654f83 (rebase -m: fix --signoff
with conflicts, 2024-04-18) where Git started writing conflict comments
to the file at 'rebase_path_message()'.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>