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