Merge branch 'jk/diff-no-index-with-pathspec'

"git diff --no-index dirA dirB" can limit the comparison with
pathspec at the end of the command line, just like normal "git
diff".

* jk/diff-no-index-with-pathspec:
  diff --no-index: support limiting by pathspec
  pathspec: add flag to indicate operation without repository
  pathspec: add match_leading_pathspec variant