add some allowed GIT_ env vars

ENV_ASKPASS/USER/PASS are documented via man gitcredentials, and are
used among other things by jenkins to pass credentials down to git for
authentication.

https://github.com/jenkinsci/git-plugin/blob/1e3488a730a169778ba0863dd4edbb1dc29154a1/README.adoc#git-bindings
https://github.com/jenkinsci/git-plugin/blob/9429e7d05df3dbb4060ac6ab4da6538bb0eb50ba/src/main/java/jenkins/plugins/git/GitUsernamePasswordBinding.java#L130

pre_commit/git.py

@@ -43,6 +43,7 @@ def no_git_env(_env: Mapping[str, str] | None = None) -> dict[str, str]:
             'GIT_SSL_NO_VERIFY', 'GIT_CONFIG_COUNT',
             'GIT_HTTP_PROXY_AUTHMETHOD',
             'GIT_ALLOW_PROTOCOL',
+            'SSH_ASKPASS', 'GIT_ASKPASS', 'GIT_USERNAME', 'GIT_PASSWORD',
         }
     }