Add full parser for git diff-tree output

Add full parser for git diff-tree output

The -c option to git log/diff-tree produces "merged" diffs with a
different format from normal ones.  However, the existing diff-tree
parser only accepted non-merged diff lines.  Merged diff lines caused
the parser to get out of sync.  This patch adds a full parser for diffs
with any number of parents.  See the "DIFF FORMAT FOR MERGES" section in
the git-diff-tree man page for more information.

Signed-off-by: Brian Downing <bdowning@lavos.net>