diff --git a/tools/lib/git-tools.bash b/tools/lib/git-tools.bash
new file mode 100644
index 0000000000..0aa0f6ad8d
--- /dev/null
+++ b/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
+}