commit: make --only --allow-empty work without paths

--only is implied when paths are present, and required
them unless --amend. But with --allow-empty it should
be allowed as well - it is the only way to create an
empty commit in the presence of staged changes.

Signed-off-by: Andreas Krey <a.krey@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -263,7 +263,8 @@ FROM UPSTREAM REBASE" section in linkgit:git-rebase[1].)
 	If this option is specified together with `--amend`, then
 	no paths need to be specified, which can be used to amend
 	the last commit without committing changes that have
-	already been staged.
+	already been staged. If used together with `--allow-empty`
+	paths are also not required, and an empty commit will be created.
 
 -u[<mode>]::
 --untracked-files[=<mode>]::

builtin/commit.c

@@ -1201,7 +1201,7 @@ static int parse_and_validate_options(int argc, const char *argv[],
 
 	if (also + only + all + interactive > 1)
 		die(_("Only one of --include/--only/--all/--interactive/--patch can be used."));
-	if (argc == 0 && (also || (only && !amend)))
+	if (argc == 0 && (also || (only && !amend && !allow_empty)))
 		die(_("No paths with --include/--only does not make sense."));
 	if (argc == 0 && only && amend)
 		only_include_assumed = _("Clever... amending the last one with dirty index.");

t/t7501-commit.sh

@@ -155,6 +155,15 @@ test_expect_success 'amend --only ignores staged contents' '
 	git diff --exit-code
 '
 
+test_expect_success 'allow-empty --only ignores staged contents' '
+	echo changed-again >file &&
+	git add file &&
+	git commit --allow-empty --only -m "empty" &&
+	git cat-file blob HEAD:file >file.actual &&
+	test_cmp file.expect file.actual &&
+	git diff --exit-code
+'
+
 test_expect_success 'set up editor' '
 	cat >editor <<-\EOF &&
 	#!/bin/sh