Git-style Mercurial diffs lack any useful information for specifying the

revisions of files. This meant that Review Board couldn't normally

extract any useful identifying information for any file fetches. We had

worked around this in the past by utilizing the diff upload API's

base_commit_id field, but that still meant that some information was

missing.
This triggered a breakage whenever a change consisted of a moved file.

Since the revision information wasn't available, the original file

couldn't be fetched correctly, since the base_commit_id wasn't enough

to help look it up.
Fortunately, Review Board does include support for extracting special

Mercurial headers containing this information. Those are only natively

generated in an hg export (see

https://bz.mercurial-scm.org/show_bug.cgi?id=6400), but adding this to

RBTools-generated Mercurial diffs is trivial.
This change implements these headers when generating diffs. We now

always generate a Git-style diff (which was effectively the behavior

for any modern version of Review Board anyway) and prepend these headers

to it.
All diff-related unit tests have been rewritten to carefully ensure that

we're generating these diffs correctly and that they have the exact

output we expect.

Unit tests passed.
Verified that generated diffs contained revision information and that

Review Board successfully used that to locate files, even when modifying

changes moved in a parent diff.