git-commit: finishing touches.

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>