request: add git grep -o option

[Joe Perches <joe@xxxxxxxxxxx>]

Instead of duplicating a grep pattern with grep -oh after
git grep like
this:

$ git grep -P '(?:[a-z0-9_]+_)?for_each(?:_[a-z0-9_]+)?' -- '*.[ch]' | \
  grep -P -oh '(?:[a-z0-9_]+_)?for_each(?:_[a-z0-9_]+)?'

it could be useful to have git grep have the same '-o' option 
as gnu grep.