rev-parse: better error message for ambiguous arguments

Currently, if git-rev-parse encounters an argument that is neither a
recognizable revision name nor the name of an existing file or
directory, and it hasn't encountered a "--" argument, it prints an
error message saying "No such file or directory".  This can be
confusing for users, including users of programs such as gitk that
use git-rev-parse, who may then think that they can't ask about the
history of files that no longer exist.

This makes it print a better error message, one that points out the
ambiguity and tells the user what to do to fix it.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

rev-parse.c

@@ -160,6 +160,14 @@ static int show_file(const char *arg)
 	return 0;
 }
 
+static void die_badfile(const char *arg)
+{
+	if (errno != ENOENT)
+		die("'%s': %s", arg, strerror(errno));
+	die("'%s' is ambiguous - revision name or file/directory name?\n"
+	    "Please put '--' before the list of filenames.", arg);
+}
+
 int main(int argc, char **argv)
 {
 	int i, as_is = 0, verify = 0;
@@ -176,7 +184,7 @@ int main(int argc, char **argv)
 		if (as_is) {
 			if (show_file(arg) && as_is < 2)
 				if (lstat(arg, &st) < 0)
-					die("'%s': %s", arg, strerror(errno));
+					die_badfile(arg);
 			continue;
 		}
 		if (!strcmp(arg,"-n")) {
@@ -343,7 +351,7 @@ int main(int argc, char **argv)
 		if (verify)
 			die("Needed a single revision");
 		if (lstat(arg, &st) < 0)
-			die("'%s': %s", arg, strerror(errno));
+			die_badfile(arg);
 	}
 	show_default();
 	if (verify && revs_count != 1)