Merge branch 'jk/stash-disable-renames-internally'

When diff.renames configuration is on (and with Git 2.9 and later,
it is enabled by default, which made it worse), "git stash"
misbehaved if a file is removed and another file with a very
similar content is added.

* jk/stash-disable-renames-internally:
stash: prefer plumbing over git-diff

diff --cc git-stash.sh
Simple merge

diff --cc t/t3903-stash.sh
Simple merge