Merge branch 'kw/commit-keep-index-when-pre-commit-is-not-run'

"git commit" used to discard the index and re-read from the filesystem
just in case the pre-commit hook has updated it in the middle; this
has been optimized out when we know we do not run the pre-commit hook.

* kw/commit-keep-index-when-pre-commit-is-not-run:
  commit: skip discarding the index if there is no pre-commit hook

diff --cc builtin/commit.c
Simple merge