Merge branch 'nd/diff-with-path-params'

A few options of "git diff" did not work well when the command was
run from a subdirectory.

* nd/diff-with-path-params:
diff: make -O and --output work in subdirectory
diff-no-index: do not take a redundant prefix argument

diff --cc builtin/am.c
Simple merge

diff --cc builtin/diff.c
Simple merge

diff --cc diff-no-index.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge

diff --cc revision.c
Simple merge