Merge branch 'jc/doc-diff-blobs'

"git diff <blob> <blob>" was not documented and was only hinted as
an extension to "git diff <commit> <commit> -- <pathspec>", but
comparison between two blobs are more special than that.  It does
not take any pathspec to begin with.

* jc/doc-diff-blobs:
  Documentation: Describe "git diff <blob> <blob>" separately