push: add shorthand for --force-with-lease branch creation

Allow the empty string to stand in for the null SHA-1 when pushing a new
branch, like we do when deleting branches.

This means that the following command ensures that `new-branch` is
created on the remote (that is, is must not already exist):

	git push --force-with-lease=new-branch: origin new-branch

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.txt

@@ -201,7 +201,8 @@ if it is going to be updated, by requiring its current value to be
 the same as the specified value `<expect>` (which is allowed to be
 different from the remote-tracking branch we have for the refname,
 or we do not even have to have such a remote-tracking branch when
-this form is used).
+this form is used).  If `<expect>` is the empty string, then the named ref
+must not already exist.
 +
 Note that all forms other than `--force-with-lease=<refname>:<expect>`
 that specifies the expected current value of the ref explicitly are

remote.c

@@ -2304,6 +2304,8 @@ int parse_push_cas_option(struct push_cas_option *cas, const char *arg, int unse
 	entry = add_cas_entry(cas, arg, colon - arg);
 	if (!*colon)
 		entry->use_tracking = 1;
+	else if (!colon[1])
+		hashclr(entry->expect);
 	else if (get_sha1(colon + 1, entry->expect))
 		return error("cannot parse expected object name '%s'", colon + 1);
 	return 0;

t/t5533-push-cas.sh

@@ -191,4 +191,30 @@ test_expect_success 'cover everything with default force-with-lease (allowed)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'new branch covered by force-with-lease (explicit)' '
+	setup_srcdst_basic &&
+	(
+		cd dst &&
+		git branch branch master &&
+		git push --force-with-lease=branch: origin branch
+	) &&
+	git ls-remote dst refs/heads/branch >expect &&
+	git ls-remote src refs/heads/branch >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'new branch already exists' '
+	setup_srcdst_basic &&
+	(
+		cd src &&
+		git checkout -b branch master &&
+		test_commit c
+	) &&
+	(
+		cd dst &&
+		git branch branch master &&
+		test_must_fail git push --force-with-lease=branch: origin branch
+	)
+'
+
 test_done