revert: do not pass non-literal string as format to git_path()

This fixes the following warning.

    CC builtin/revert.o
builtin/revert.c: In function ‘write_cherry_pick_head’:
builtin/revert.c:311: warning: format not a string literal and no format arguments

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -308,7 +308,7 @@ static void write_cherry_pick_head(struct commit *commit, const char *pseudoref)
 
 	strbuf_addf(&buf, "%s\n", sha1_to_hex(commit->object.sha1));
 
-	filename = git_path(pseudoref);
+	filename = git_path("%s", pseudoref);
 	fd = open(filename, O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"), filename);