diff-ni: allow running from a subdirectory.
diff-ni: allow running from a subdirectory.

When run from a subdirectory of a repository, the command forgot
to adjust paths given to it with prefix.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git