Merge branch 'jc/refactor-diff-stdin'

Merge branch 'jc/refactor-diff-stdin'

Due to the way "git diff --no-index" is bolted onto by touching the
low level code that is shared with the rest of the "git diff" code,
even though it has to work in a very different way, any comparison
that involves a file "-" at the root level incorrectly tried to read
from the standard input.  This cleans up the no-index codepath
further to remove code that reads from the standard input from the
core side, which is never necessary when git is running its usual
diff operation.

* jc/refactor-diff-stdin:
  diff-index.c: "git diff" has no need to read blob from the standard input
  diff-index.c: unify handling of command line paths
  diff-index.c: do not pretend paths are pathspecs