Fix initial commit page & binary munching problem (#13249) (#13258)

Fix initial commit page & binary munching problem (#13249) (#13258)

Backport #13249

* Fix initial commit page

Unfortunately as a result of properly fixing ParsePatch the hack that
used git show <initial_commit_id> to get the diff for this failed.

This PR fixes this using the "super-secret" empty tree ref to make the
diff against.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Also fix #13248

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Update services/gitdiff/gitdiff.go

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: 6543 <6543@obermui.de>