unset git global configs

git is highly configurable and esp. for the `log` command its very
popular to modify the config to the users needs/preferences.

Unfortunatly these settings are also active in scripted environments
like tar_scm/obs_scm and can cause various errors.

This patch sets the ENV VAR GIT_CONFIG_GLOBAL to /dev/null so configs
in ~/.gitconfig and $XDG_CONFIG_HOME/git/config should be ignored.

TarSCM/scm/git.py

@@ -3,6 +3,7 @@
 import re
 import sys
 import shutil
+import tempfile
 
 from TarSCM.scm.base import Scm
 from TarSCM.exceptions import GitError
@@ -24,6 +25,15 @@ class Git(Scm):
     scm = 'git'
     _stash_pop_required = False
     partial_clone = False
+    temp_git_config_global = tempfile.NamedTemporaryFile(delete=False)
+
+    def __init__(self, args, task):
+        super().__init__(args, task)
+        os.putenv('GIT_CONFIG_GLOBAL', self.temp_git_config_global.name)
+
+    def __del__(self):
+        super().__del__()
+        os.unlink(self.temp_git_config_global.name)
 
     def _get_scm_cmd(self):
         """Compose a GIT-specific command line using http proxies"""
@@ -228,7 +238,9 @@ def update_cache(self):
         self.auth_url()
         try:
             self.helpers.safe_run(
-                self._get_scm_cmd() + ['config', 'remote.origin.url',
+                self._get_scm_cmd() + ['config',
+                                       '--local',
+                                       'remote.origin.url',
                                        self.url],
                 cwd=self.clone_dir,
                 interactive=sys.stdout.isatty()