Use modern Python exception syntax

"except Exception as e" instead of "except Exception, e"

This is part of a transition to supporting Python 3.  Python >= 2.6
support "as" syntax.

Note: this removes Python 2.5 support.

Change-Id: I309599f3981bba2b46111c43102bee38ff132803

git_command.py

@@ -217,7 +217,7 @@ class GitCommand(object):
                            stdin = stdin,
                            stdout = stdout,
                            stderr = stderr)
-    except Exception, e:
+    except Exception as e:
       raise GitError('%s: %s' % (command[1], e))
 
     if ssh_proxy: