Merge branch 'jc/grep-f-relative-to-cwd'

"cd sub && git grep -f patterns" tried to read "patterns" file at
the top level of the working tree; it has been corrected to read
"sub/patterns" instead.

* jc/grep-f-relative-to-cwd:
  grep: -f <path> is relative to $cwd

diff --cc builtin/grep.c
Simple merge

diff --cc t/t7810-grep.sh
Simple merge