Merge branch 'jc/refactor-diff-stdin' into maint

Merge branch 'jc/refactor-diff-stdin' into maint

"git diff", "git status" and anything that internally uses the
comparison machinery was utterly broken when the difference
involved a file with "-" as its name.  This was due to the way "git
diff --no-index" was incorrectly bolted on to the system, making
any comparison that involves a file "-" at the root level
incorrectly read from the standard input.

* 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