Fix bug where colons in paths raise a ValueError on diff() calls.

This commit introduces a potential fix for #1490 and #1483, in which an
`invalid literal for int() with base 10: 'n'` exception was raised
within a diff operation. Within `_handle_diff_line()`, we split the
output of `git diff-tree` on colons (`:` characters), under the
assumption that there are no colons within the paths of the files being
diffed. On POSIX systems this is not a valid assumption. The fix is to
split on `\x00:`, since a null character always precedes the colons we
actually need to split on.

A test already existed for this case (`test_diff_file_with_colon()`),
but it was marked as skipped.

* Split on `\x00:` instead of `:` in `_handle_diff_line()`.
* Unskip `test_diff_file_with_colon()`.

git/diff.py

@@ -570,7 +570,10 @@ def _index_from_patch_format(cls, repo: "Repo", proc: Union["Popen", "Git.AutoIn
     def _handle_diff_line(lines_bytes: bytes, repo: "Repo", index: DiffIndex) -> None:
         lines = lines_bytes.decode(defenc)
 
-        for line in lines.split(":")[1:]:
+        # Discard everything before the first colon, and the colon itself.
+        _, _, lines = lines.partition(":")
+
+        for line in lines.split("\x00:"):
             meta, _, path = line.partition("\x00")
             path = path.rstrip("\x00")
             a_blob_id: Optional[str]

test/test_diff.py

@@ -236,7 +236,6 @@ def test_diff_index_raw_format(self):
             res[0].b_path,
         )
 
-    @unittest.skip("This currently fails and would need someone to improve diff parsing")
     def test_diff_file_with_colon(self):
         output = fixture("diff_file_with_colon")
         res = []