Merge branch 'mt/grep-submodules-working-tree'

"git grep --recurse-submodules" that looks at the working tree
files looked at the contents in the index in submodules, instead of
files in the working tree.

* mt/grep-submodules-working-tree:
grep: fix worktree case in submodules

diff --cc builtin/grep.c
Simple merge

diff --cc t/t7814-grep-recurse-submodules.sh
Simple merge