Merge branch 'js/expand-runtime-prefix'

Pathname expansion (like "~username/") learned a way to specify a
location relative to Git installation (e.g. its $sharedir which is
$(prefix)/share), with "%(prefix)".

* js/expand-runtime-prefix:
  expand_user_path: allow in-flight topics to keep using the old name
  interpolate_path(): allow specifying paths relative to the runtime prefix
  Use a better name for the function interpolating paths
  expand_user_path(): clarify the role of the `real_home` parameter
  expand_user_path(): remove stale part of the comment
  tests: exercise the RUNTIME_PREFIX feature

cache.h

@@ -1255,7 +1255,9 @@ typedef int create_file_fn(const char *path, void *cb);
 int raceproof_create_file(const char *path, create_file_fn fn, void *cb);
 
 int mkdir_in_gitdir(const char *path);
-char *expand_user_path(const char *path, int real_home);
+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)
 {