[PATCH] Do not run useless show-diff on unmerged paths repeatedly.

[PATCH] Do not run useless show-diff on unmerged paths repeatedly.

When run on unmerged dircache, show-diff compares the working
file with each non-empty stage for that path.  Two out of three
times, this is not very helpful.  This patch makes it report the
unmergedness only once per each path and avoids running the
actual diff.

Upper layer SCMs like Cogito are expected to find out mode/SHA1
for each stage by using "show-files --stage" and run the diff
itself.  This would result in more sensible diffs.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>