Define FileHeader to parse the header block of a git diff

Define FileHeader to parse the header block of a git diff

This class parses the top header lines of a git style diff, such as:

  diff --git a/SUBMITTING_PATCHES b/Q
  similarity index 100%
  copy from SUBMITTING_PATCHES
  copy to Q

or:

  diff --git a/Q b/Q
  new file mode 100644
  index 0000000..e4a135e
  --- /dev/null
  +++ b/Q

and makes the information available in an object form.  Unit tests
cover the different styles of headers that are commonly created by
C git, including both rename formats.

The hunk header information is not handled by this class, and it
does not have a public API.  It is my intention to wrap this into
a larger container class that handles multiple FileHeaders at once,
with the base case of course being a single FileHeader describing
a patch that impacts only one file.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>