When a git change includes copied files and we have exclude patterns set

(either via the config file or the command-line), we were not parsing

the git diff-tree output correctly.
This change redoes the parsing to use the -z flag, which splits

records with NUL characters instead of tabs (making us more resilient

with filenames). According to the diff-tree docs, the resulting records

may include either one or two files based on the change marker. If we

have a "C" (copy) or "R" (rename) change, we use the second filename as

the test for whether we should exclude the result from the diff.

Otherwise, we parse out a single filename and test that.
A new unit test has been added which failed with the initial

implementation and succeeds with the new one.

Verified that diffs were properly created with the reported situation.
Ran unit tests.