git-commit: log parameter updates.

While moving '-m' to make room for CVS compatible "here is the
log message", enhance source of log parameters.

  -m 'message': a command line parameter.
  -F <file>   : a file (use '-' to read from stdin).
  -C <commit> : message in existing commit.
  -c <commit> : message in existing commit (allows further editing).

Longer option names for these options are also available.

While we are at it, get rid of shell array bashism.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-rebase-script

@@ -37,7 +37,7 @@ do
 	esac
 	S=`cat "$GIT_DIR/HEAD"` &&
         GIT_EXTERNAL_DIFF=git-apply-patch-script git-diff-tree -p $commit &&
-	git-commit-script -m "$commit" || {
+	git-commit-script -C "$commit" || {
 		echo $commit >>$fail
 		git-read-tree --reset -u $S
 	}