Merge branch 'jk/submodule-remote-lookup-cleanup' into jch

Updating submodules from the upstream did not work well when
submodule's HEAD is detached, which has been improved.

* jk/submodule-remote-lookup-cleanup:
  submodule: look up remotes by URL first
  submodule: move get_default_remote_submodule()
  submodule--helper: improve logic for fallback remote name
  remote: remove the_repository from some functions
  dir: move starts_with_dot(_dot)_slash to dir.h
  remote: fix tear down of struct remote
  remote: remove branch->merge_name and fix branch_release()

diff --cc builtin/pull.c
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc t/t7406-submodule-update.sh
Simple merge