ui-diff: Use diff_tree_sha1() for raw diff formatting

Use Git's internal diff_tree_sha1() function for the /rawdiff/ command
instead of trying to recreate this functionality.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
ui-shared: Drop filepair_cb_raw() and helper

Remove filepair_cb_raw() and all related functions. These are no longer
needed. We now use Git's internal functions for raw diff formatting
everywhere.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>