No commit history issue

dvc/git_wrapper.py

@@ -427,13 +427,20 @@ class GitWrapper(GitWrapperI):
     @staticmethod
     def git_prev_commit():
         cmd = 'git check-ref-format --branch @{-1}'
-        return Executor.exec_cmd_only_success(cmd.split()).strip()
+        code, out, err = Executor.exec_cmd(cmd.split())
+        if code != 0:
+            return ''
+        return out
 
     @staticmethod
     def state_files_for_previous_commit():
         commit = GitWrapper.git_prev_commit()
         Logger.debug(u'[dvc-git] Previous commit "{}"'.format(commit))
 
+        if not commit:
+            Logger.debug(u'[dvc-git] Previous commit does not exist')
+            return []
+
         git_cmd = u'git ls-tree --name-only -r {}'.format(commit)
         lines = Executor.exec_cmd_only_success(git_cmd.split()).split('\n')
         return filter(lambda s: s.startswith(ConfigI.STATE_DIR), lines)