Strip a/ and b/ prefixes for rename-only and mode-only git diffs

[afonsojanu]

A git diff that only renames a file, or only changes its mode, has no
content to show, so git omits both the index <sha>..<sha> line and the
unified ---/+++ hunk lines. For example:

diff --git a/myfile2.txt b/myfile3.txt
similarity index 100%
rename from myfile2.txt
rename to myfile3.txt

parse_header() normally relies on one of those two things being present
to recognize the diff as git-flavored and route it to parse_git_header,
which knows to strip the a//b/ prefixes. When neither is there, the
generic diffcmd_header regex ends up matching the diff --git a/x b/y
line instead, since that pattern happens to fit it too, and the raw,
prefixed paths leak straight into old_path/new_path:

>>> whatthepatch.parse_patch(text)[0].header
header(index_path=None, old_path='a/myfile2.txt', old_version=None, new_path='b/myfile3.txt', new_version=None)

This fixes it two ways:

• parse_diff_header now checks the git-specific diff --git a/... b/...
  line ahead of the generic one, so it's always routed to
  parse_git_header.
• parse_git_header's fallback (which uses the paths straight from the
  diff --git command line when nothing more specific was found) no
  longer requires blob hashes to be present, since a pure rename or mode
  change never has them.

Added two regression tests with fixtures for a rename-only diff and a
mode-only diff, both of which fail against the old code with the a//b/
prefixes still attached and pass with the fix. The rest of the suite
(tests/test_patch.py and tests/test_apply.py) still passes locally.

Fixes #43.

[afonsojanu]

Part of a general cleanup on my end, closing this along with the rest of what I had open.