There is no reason to reject initial dashes in git-grep
expressions... other than the code not supporting it previously.
A new method is introduced to relax the security checks.
It is a waste of resources to scan them looking for matches
because they are never returned back - they appear as empty
lines in the current format.
Notably, even if they were returned, it is unlikely that matching
in binary files makes sense when the goal is "code search".
- The parser of `git grep`'s output uses `bufio.Scanner`, which is a good
choice overall, however it does have a limit that's usually not noticed,
it will not read more than `64 * 1024` bytes at once which can be hit in
practical scenarios.
- Use `bufio.Reader` instead which doesn't have this limitation, but is
a bit harder to work with as it's a more lower level primitive.
- Adds unit test.
- Resolves https://codeberg.org/forgejo/forgejo/issues/3149
