commit: use value of GIT_REFLOG_ACTION env variable as reflog message

commit: use value of GIT_REFLOG_ACTION env variable as reflog message

The environment variable GIT_REFLOG_ACTION was used by git-commit.sh,
but when it was converted to a builtin
(f5bbc3225c4b073a7ff3218164a0c820299bc9c6, Port git commit to C,
Nov 8 2007) this was lost.

Let's use it again as it is more user friendly when reverting or
cherry-picking to see "revert" or "cherry-pick" in the reflog rather
than to just see "commit".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>