bisect-kit: fix pylint warnings

depot_tools' pylint uprev to 2.6

BUG=None
TEST=pylint

Change-Id: Ia1b8dfb6304de60d7235d86abf4ba728252fd56d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2557857
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
diff --git a/bisect_kit/git_util.py b/bisect_kit/git_util.py
index 9d7ea3c..3f8f4f5 100644
--- a/bisect_kit/git_util.py
+++ b/bisect_kit/git_util.py
@@ -489,10 +489,10 @@
     output = util.check_output(
         'git', 'rev-parse', '%s^{commit}' % rev, '--', cwd=git_repo)
     git_rev = output.rstrip('-\n')
-  except subprocess.CalledProcessError:
+  except subprocess.CalledProcessError as e:
     # Do not use 'git rev-parse --disambiguate' to determine uniqueness
     # because it searches objects other than commits as well.
-    raise ValueError('%s is not unique or does not exist' % rev)
+    raise ValueError('%s is not unique or does not exist' % rev) from e
   assert is_git_rev(git_rev)
   return git_rev