MAINT: git security shim for 1.9.x

* replicated scipygh-16139 on the latest maintenance branch
because the `master` branch of the wheels repo will
encounter the issues described in that PR (for example, see:
MacPython/scipy-wheels#166 which
has Travis and Azure failures caused by those same
versioning issues)

* I think the `cwd` is still correct even though the patch
is being applied to a different file this time (used to be
`setup.py`), though we could double check this by pointing
the wheels PR at the commit hash of this PR if we want

* any reason not to forward port this as well at this point,
if we're going to need to keep backporting it?

tools/version_utils.py

@@ -81,7 +81,9 @@ def _minimal_ext_cmd(cmd):
         return out
 
     try:
-        out = _minimal_ext_cmd(['git', 'rev-parse', 'HEAD'])
+        cwd = os.getcwd()
+        git_dir = os.path.join(cwd, ".git")
+        out = _minimal_ext_cmd(['git', '--git-dir', git_dir, 'rev-parse', 'HEAD'])
         GIT_REVISION = out.strip().decode('ascii')[:7]
 
         # We need a version number that's regularly incrementing for newer commits,