git-merge-cache -o does all the automerges in a single shot

When you pass git-merge-cache the -o option, it tries to do all the
automatic merges and possibly return error if any of them failed, instead
of the default behaviour of failing immediately after the first failed
automatic merge.

Ported from the Cogito branch - Cogito needs this behaviour.

merge-cache.c

@@ -5,6 +5,7 @@
 
 static const char *pgm = NULL;
 static const char *arguments[8];
+static int one_shot;
 static int err;
 
 static void run_program(void)
@@ -25,8 +26,12 @@ static void run_program(void)
 			    NULL);
 		die("unable to execute '%s'", pgm);
 	}
-	if (waitpid(pid, &status, 0) < 0 || !WIFEXITED(status) || WEXITSTATUS(status))
+	if (waitpid(pid, &status, 0) < 0 || !WIFEXITED(status) || WEXITSTATUS(status)) {
+		if (one_shot)
 			err++;
+		else
+			die("merge program failed");
+	}
 }
 
 static int merge_entry(int pos, const char *path)
@@ -92,12 +97,17 @@ int main(int argc, char **argv)
 	int i, force_file = 0;
 
 	if (argc < 3)
-		usage("merge-cache <merge-program> (-a | <filename>*)");
+		usage("merge-cache [-o] <merge-program> (-a | <filename>*)");
 
 	read_cache();
 
-	pgm = argv[1];
-	for (i = 2; i < argc; i++) {
+	i = 1;
+	if (!strcmp(argv[1], "-o")) {
+		one_shot = 1;
+		i++;
+	}
+	pgm = argv[i++];
+	for (; i < argc; i++) {
 		char *arg = argv[i];
 		if (!force_file && *arg == '-') {
 			if (!strcmp(arg, "--")) {