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()`.

That way people who use it won't be deterred, while it unifies style
everywhere.

This reverts commit 01f09888208341876d1480bd22dc8f4107c100f1.

Use NUL character instead of semicolon to extract meta and path. Avoid errors in during git diff when dealing with filenames containing semicolons