optionally use select_related to pull related fields for old

[dgravitate]

Add the ability to use select_related to pull related fields when getting the old instance to compare against. This should eliminate some unnecessary queries and boost performance, particularly when dealing with lots of related fields.

[codecov]

Codecov Report

Merging #415 (b8d6dab) into master (a00d2c2) will decrease coverage by 0.77%.
The diff coverage is 53.84%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   91.79%   91.01%   -0.78%     
==========================================
  Files          23       23              
  Lines         646      657      +11     
==========================================
+ Hits          593      598       +5     
- Misses         53       59       +6     

Impacted Files	Coverage Δ
auditlog/receivers.py	86.36% <50.00%> (-13.64%)	⬇️
auditlog/conf.py	100.00% <100.00%> (ø)

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[hramezani]

Thanks @dgravitate for the patch 👍

I haven't looked in detail at the patch. but you need:

• Add tests for the patch
• Document the new setting in settings section of https://github.com/jazzband/django-auditlog/blob/master/docs/source/usage.rst
• Add a change log entry in https://github.com/jazzband/django-auditlog/blob/master/CHANGELOG.md