This adds a new HgGitDiffParser which will parse the headers generated
by "hg export --git" and then rely on the GitDiffParser to parse the
diff contents.

The logic for deciding which hg parser to use has also been changed
slightly so that it takes into account the possible presence of the
commented headers hg provides.

Tests for mercurial git diff parsing have been updated to provide the
proper extended headers.

New tests, unit tests pass, uploaded diff from "hg export --git" parses and displays.