Merge branch 'nd/worktree-kill-parse-ref'

"git gc" did not interact well with "git worktree"-managed
per-worktree refs.

* nd/worktree-kill-parse-ref:
  refs: kill set_worktree_head_symref()
  worktree.c: kill parse_ref() in favor of refs_resolve_ref_unsafe()
  refs: introduce get_worktree_ref_store()
  refs: add REFS_STORE_ALL_CAPS
  refs.c: make submodule ref store hashmap generic
  environment.c: fix potential segfault by get_git_common_dir()

refs/refs-internal.h

@@ -482,6 +482,10 @@ struct ref_store;
 #define REF_STORE_WRITE		(1 << 1) /* can perform update operations */
 #define REF_STORE_ODB		(1 << 2) /* has access to object database */
 #define REF_STORE_MAIN		(1 << 3)
+#define REF_STORE_ALL_CAPS	(REF_STORE_READ | \
+				 REF_STORE_WRITE | \
+				 REF_STORE_ODB | \
+				 REF_STORE_MAIN)
 
 /*
  * Initialize the ref_store for the specified gitdir. These functions