BLD, MAINT: git security/version shim

* this is an attempt to deal with the new
security measure in git:
https://github.blog/2022-04-12-git-security-vulnerability-announced/

* it has been blocking the release of SciPy 1.8.1
and NumPy point release for some time

* I'm going to try to point the problem wheels
repo PR at the hash of this commit/branch before
merging if possible:
MacPython/scipy-wheels#167

* based on feedback from Henry over here, this does
seem to help locally:
pypa/manylinux#1309 (comment)

setup.py

@@ -79,7 +79,9 @@ 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,