tools: Add a require_clean_work_tree function, from Git upstream.

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.

tools/lib/git-tools.bash

@@ -0,0 +1,35 @@
+# shellcheck shell=bash
+
+# Borrowed from Git's git-sh-setup.
+#
+# See git.git commit 92c62a3f4 (from 2010!); as of 2020 with Git 2.26,
+# this function has only needed one edit since then, adding localization
+# with gettext, which we can omit.
+require_clean_work_tree () {
+    git rev-parse --verify HEAD >/dev/null || exit 1
+    git update-index -q --ignore-submodules --refresh
+    err=0
+
+    if ! git diff-files --quiet --ignore-submodules
+    then
+        echo >&2 "Cannot $1: You have unstaged changes."
+        err=1
+    fi
+
+    if ! git diff-index --cached --quiet --ignore-submodules HEAD --
+    then
+        if [ $err = 0 ]
+        then
+            echo >&2 "Cannot $1: Your index contains uncommitted changes."
+        else
+            echo >&2 "Additionally, your index contains uncommitted changes."
+        fi
+        err=1
+    fi
+
+    if [ $err = 1 ]
+    then
+        test -n "$2" && echo >&2 "$2"
+        exit 1
+    fi
+}