Merge branch 'mt/threaded-grep-in-object-store' into pu

Traditionally, we avoided threaded grep while searching in objects
(as opposed to files in the working tree) as accesses to the object
layer is not thread-safe.  This limitation is getting lifted.

* mt/threaded-grep-in-object-store:
  grep: move driver pre-load out of critical section
  grep: re-enable threads in non-worktree case
  grep: protect packed_git [re-]initialization
  grep: allow submodule functions to run in parallel
  submodule-config: add skip_if_read option to repo_read_gitmodules()
  grep: replace grep_read_mutex by internal obj read lock
  object-store: allow threaded access to object reading
  replace-object: make replace operations thread-safe
  grep: fix racy calls in grep_objects()
  grep: fix race conditions at grep_submodule()
  grep: fix race conditions on userdiff calls

diff --cc Documentation/git-grep.txt
Simple merge

diff --cc builtin/grep.c
Simple merge

diff --cc grep.c
Simple merge

diff --cc grep.h
Simple merge

diff --cc object.c
Simple merge

diff --cc packfile.c
Simple merge

diff --cc sha1-file.c
Simple merge

diff --cc submodule-config.c
Simple merge

diff --cc unpack-trees.c
Simple merge