git grep file name only

The error message helps

$ git grep 'foo' --files-with-matches 
fatal: option '--files-with-matches' must come before non-option arguments

$ git grep --files-with-matches 'foo'
<list of matching files>

To count the words, this is how I'd do it with GNU grep, not sure if git grep has relevant options

$ grep --exclude-dir=.git -RowF 'foo' | wc -l
717

From man grep

-R, --dereference-recursive Read all files under each directory, recursively. Follow all symbolic links, unlike -r.

-o, --only-matching Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.

-w, --word-regexp Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.

-F, --fixed-strings Interpret PATTERN as a list of fixed strings (instead of regular expressions), separated by newlines, any of which is to be matched.

--exclude-dir=DIR Exclude directories matching the pattern DIR from recursive searches.