Swallow git import exception

See the following issue which explains why we can't catch a specific exception type:

gitpython-developers/GitPython#637

Author: Roey Darwish Dror

Diff for: backslash/contrib/slash_plugin.py

@@ -13,7 +13,11 @@
 import logbook
 import requests
 
-import git
+try:
+    import git
+except Exception as e:
+    pass
+
 import slash
 from sentinels import NOTHING
 from slash import config as slash_config