Fix GetFilesChangedBetween if the file name may be escaped (#23272) (#23278)

Fix GetFilesChangedBetween if the file name may be escaped (#23272) (#23278)

Backport #23272

The code for GetFilesChangedBetween uses `git diff --name-only
base..head` to get the names of files changed between base and head
however this forgets that git will escape certain values.

This PR simply switches to use `-z` which has the `NUL` character as the
separator.

Ref https://github.com/go-gitea/gitea/pull/22568#discussion_r1123138096

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>