git-commit: finishing touches.
Introduce --only flag to allow the new "partial commit"
semantics when paths are specified. The default is still the
traditional --include semantics. Once peoples' fingers and
scripts that want the traditional behaviour are updated to
explicitly say --include, we could change it to either default
to --only, or refuse to operate without either --only/--include
when paths are specified.
This also fixes a couple of bugs in the previous round. Namely:
- forgot to save/restore index in some cases.
- forgot to use the temporary index to show status when '--only
paths...' semantics was used.
- --author did not take precedence when reusing an existing
commit.
Signed-off-by: Junio C Hamano <junkio@cox.net>