diff-files: -1/-2/-3 to diff against unmerged stage.

diff-files: -1/-2/-3 to diff against unmerged stage.

While resolving conflicted merge, it was not easy to compare the
working tree file with unmerged index entries.  This commit
introduces new options -1/-2/-3 (with synonyms --base, --ours,
and --theirs) to compare working tree files with specified
stages.

When none of these options are given, the command defaults to -2
if the index file is unmerged, otherwise it acts as before.

[jc: majorly butchered from the version Linus originally posted.]

Signed-off-by: Junio C Hamano <junkio@cox.net>