Replaces count by exists in populate_history command

[fabiocapsouza]

Description

Replaces a count() query in populate_history command by exists()

Related Issue

#922

Motivation and Context

The populate_history command checks if historical records exist for each model. However, this check uses a count query that can be expensive if the historical table is large. An exists query serves this purpose and is much faster to run.

How Has This Been Tested?

I ran all tests using the runtests.py script and sqlite3 database.

The populate_history command is already tested by TestPopulateHistory TestCase and covers this change.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #982 (ac5a83d) into master (720fd8a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #982   +/-   ##
=======================================
  Coverage   97.64%   97.64%           
=======================================
  Files          23       23           
  Lines        1147     1147           
  Branches      222      222           
=======================================
  Hits         1120     1120           
  Misses         12       12           
  Partials       15       15           

Impacted Files	Coverage Δ
...le_history/management/commands/populate_history.py	96.00% <100.00%> (ø)

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