log, diff: Add rename detection support

log, diff: Add rename detection support

Implement rename detection in the command line diff and log commands.
Also support --name-status, -p and -U flags, as these can be quite
useful to view more detail.

All of the Git patch file formatting code is now moved over to the
DiffFormatter class.  This permits us to reuse it in any context,
including inside of IDEs.

Change-Id: I687ccba34e18105a07e0a439d2181c323209d96c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>