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()