git grep does not find all occurrences on macOS

Hi!

I encountered rather subtle issue on in git 2.47.0 on macOS 14.7 (installed from Homebrew):

git grep will not find all occurrences of string patterns containing a “.” under some conditions. In my case I have an ISO-8859 encoded text file which contains umlauts. If the string I’m grepping for occurs after a non-ASCII character in this file, git grep will not find it.

I’ve put up a reproducer here https://github.com/iokill/repro-git-grep-issue, but the gist of it is "git grep quz.baz" on the ISO-8859-encoded file below will not return anything, when it should return the line "quz.baz=3":

-->8------------- foo=bar umlauts=äöü quz.baz=3 --8<-------------

From what I’ve found so far, this occurs on macOS, but not on Linux and it has to be connected to the regex matching code, because the issue does not occur with --fixed-strings or --perl-regexp.

Thanks!
- David