commit: allow core.commentChar=auto for character auto selection

When core.commentChar is "auto", the comment char starts with '#' as
in default but if it's already in the prepared message, find another
char in a small subset. This should stop surprises because git strips
some lines unexpectedly.

Note that git is not smart enough to recognize '#' as the comment char
in custom templates and convert it if the final comment char is
different. It thinks '#' lines in custom templates as part of the
commit message. So don't use this with custom templates.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -69,6 +69,7 @@ unsigned long pack_size_limit_cfg;
  * that is subject to stripspace.
  */
 char comment_line_char = '#';
+int auto_comment_line_char;
 
 /* Parallel index stat data preload? */
 int core_preload_index = 0;