config: set comment_line_str to "#" when core.commentChar=auto

If conflict comments already use a comment character that isn't "#", and
core.commentChar is set "auto", Git will ignore these lines during the
scan using ignored_log_message_bytes() and pick a new comment character
based on the rest of the message. The newly chosen character may be
different from the one used in the conflict comments and therefore,
these are no longer treated as comments and end up in the final commit
message.

For example, during a rebase if the user previously set
core.commentChar=% and then encounters a conflict, conflict comments
like "% Conflicts:" are generated. If the user subsequently sets
core.commentChar=auto before running `rebase --continue`, Git parses the
"auto" setting and begins scanning. It first uses the existing
'comment_line_str' (which is '%') to detect and ignore conflict comments
via ignored_log_message_bytes().

Then, Git scans the rest of the message (excluding conflict comments),
sees that none of the remaining lines start with '#' and decides to set
comment_line_str to '#'. Since the final commit character differs from
the one used in the conflict comments, those lines are no longer
considered comments and get included in the final commit message.

Set 'comment_line_str' to '#' when core.commentChar is set to 'auto' to
reset any previously set value.

While this does not solve the issue of conflict comment inclusion and
the user visible behaviour stays tha same, it standardizes the behaviour
of the code by always resetting 'comment_line_str' to '#' when
core.commentChar=auto is parsed.

The patch text is based on Phillip Wood's message:
https://lore.kernel.org/git/9e96aaab-79a2-4632-94cd-d016d4a63b30@gmail.com/
and the commit log message is wriiten by me.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
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>

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