Use /bin/sh for git pre-commit hook

There were no bash-specific features in this file, so eliminate
dependency on bash and use regular sh. Shellcheck tests pass with
the #!/bin/sh shebang.

scripts/git/pre-commit.git-hook

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 #
 # To install this script, copy it to .git/hooks/pre-commit in local copy of
 # the chutney git repo, and make sure it has permission to execute.