git-commit: filter out log message lines only when editor was run.

git-commit: filter out log message lines only when editor was run.

The current behaviour strips out lines starting with a # even when fed
through stdin or -m.  This is particularly bad when importing history from
another SCM (tailor 0.9.23 uses git-commit).  In the best cases all lines
are stripped and the commit fails with a confusing "empty log message"
error, but in many cases the commit is done, with loss of information.

Note that it is quite peculiar to just have "#" handled as a leading
comment char here.  One commonly meet CVS: or CG: or STG: as prefixes, and
using GIT: would be more robust as well as consistent with other commit
tools.  However, that would break any tool relying on the # (if any).

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>