Show diff on pre-commit failure

[jakob-keller]

Description of Change

Addresses #1179 (comment)

Assumptions

None

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.94%. Comparing base (96781a2) to head (e156634).
Report is 52 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1181      +/-   ##
==========================================
- Coverage   86.99%   86.94%   -0.06%     
==========================================
  Files          64       64              
  Lines        5983     5983              
==========================================
- Hits         5205     5202       -3     
- Misses        778      781       +3     

Flag	Coverage Δ
unittests	86.94% <ø> (-0.06%)	⬇️

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