Merge branch 'rs/maint-grep-F'

"git grep -e '$pattern'", unlike the case where the patterns are read from
a file, did not treat individual lines in the given pattern argument as
separate regular expressions as it should.

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 t/t7810-grep.sh
Simple merge