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()

diff --cc branch.c
Simple merge

diff --cc environment.c
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

diff --cc refs/files-backend.c
Simple merge

diff --cc refs/refs-internal.h
Simple merge

diff --cc worktree.c
Simple merge