expand_user_path: allow in-flight topics to keep using the old name

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1247,6 +1247,8 @@ int raceproof_create_file(const char *path, create_file_fn fn, void *cb);
 
 int mkdir_in_gitdir(const char *path);
 char *interpolate_path(const char *path, int real_home);
+/* NEEDSWORK: remove this synonym once in-flight topics have migrated */
+#define expand_user_path interpolate_path
 const char *enter_repo(const char *path, int strict);
 static inline int is_absolute_path(const char *path)
 {