This is verbatim from Git upstream, at an older version. (The one
change since then is to add localization for the messages like "You
have unstaged changes" -- which complicates the code, is important and
worth it for Git itself, but for our tools we can do without.)
This function will replace our use of `git diff-index --quiet HEAD`
in several scripts. The key differences in behavior are:
* The `git update-index --refresh`. Without this, on Windows
apparently `git diff-index` routinely (but not all the time!)
reports that tons of files have changed. See report:
https://chat.zulip.org/#narrow/stream/9-issues/topic/.2E.2Ftools.2Ffetch-pull-request.20issue/near/834435
* Instead of one command comparing the worktree to HEAD, we
separately compare the worktree to the index and the index to
HEAD, and abort if either diff is nonempty. This one is obvious,
but rather an edge case (it matters only if you've managed to
make the worktree and HEAD agree while the index has some
changes), and the extra code is annoying if written out in every
script that needs it. But that's what a subroutine is for. :-)
We'll make a few tweaks before actually switching to use this.
Greg Price