libgit2: updated code for tests.

- listing all commits will use only subprocess as it will return empty list for empty repos
- updated logic for empty repos so it's the best combination of speed and correctness

vcsserver/git.py

@@ -161,8 +161,15 b' class GitRemote(object):'
         repo = self._factory.repo_libgit2(wire)
 
         try:
-            return not repo.head.name
+            has_head = repo.head.name
+            if has_head:
+                return False
+
+            # NOTE(marcink): check again using more expensive method
+            return repo.is_empty
         except Exception:
+            pass
+
             return True
 
     @reraise_safe_exceptions
@@ -783,8 +790,6 b' class GitRemote(object):'
 
     @reraise_safe_exceptions
     def get_all_commit_ids(self, wire):
-        if self.is_empty(wire):
-            return []
 
         cmd = ['rev-list', '--reverse', '--date-order', '--branches', '--tags']
         try: