Merge pull request #948 from firba1/bariteau/add_git_ssh_command_support

pass GIT_SSH_COMMAND to git commands

pre_commit/git.py

@@ -29,7 +29,7 @@ def no_git_env():
     # GIT_INDEX_FILE: Causes 'error invalid object ...' during commit
     return {
         k: v for k, v in os.environ.items()
-        if not k.startswith('GIT_') or k in {'GIT_SSH'}
+        if not k.startswith('GIT_') or k in {'GIT_SSH', 'GIT_SSH_COMMAND'}
     }