git-commit: do not muck with commit message when no_edit is set.
Spotted by Linus and Darrin Thompson. When we took a commit message from -F <file> with an incomplete line, we appended "git status" output, which ended up attaching a lone "#" at the end.
We still need the "do we have anything to commit?" check by running "status" (which has to know what to do in different cases with -i/-o/-a), but there is no point appending its output to the proposed commit message given by the user.