Show usage string for 'git merge-ours -h'

This change is strictly about 'git merge-ours -h' without
any other options and arguments.

This change cannot break compatibility since merge drivers are
always passed '--', among other arguments.

Any usage string for this command is a lie, since it ignored its
arguments until now.  Still, it makes sense to let the user know
the expected usage when asked.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge-ours.c

@@ -10,6 +10,9 @@
 #include "git-compat-util.h"
 #include "builtin.h"
 
+static const char builtin_merge_ours_usage[] =
+	"git merge-ours <base>... -- HEAD <remote>...";
+
 static const char *diff_index_args[] = {
 	"diff-index", "--quiet", "--cached", "HEAD", "--", NULL
 };
@@ -17,6 +20,9 @@ static const char *diff_index_args[] = {
 
 int cmd_merge_ours(int argc, const char **argv, const char *prefix)
 {
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage(builtin_merge_ours_usage);
+
 	/*
 	 * We need to exit with 2 if the index does not match our HEAD tree,
 	 * because the current index is what we will be committing as the