PYTHON-5467 Fix codecov upload on Evergreen

[blink1073]

PYTHON-5467

Changes in this PR

Update the codecov evergreen function to handle merge commits in addition to PR commits.

Test Plan

We will need to merge the PR to see if it works.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.64%. Comparing base (b1a0a1f) to head (c6f0317).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2702   +/-   ##
=======================================
  Coverage   53.64%   53.64%           
=======================================
  Files         129      129           
  Lines       23771    23771           
  Branches     4071     4071           
=======================================
  Hits        12753    12753           
  Misses      10018    10018           
  Partials     1000     1000           

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[sleepyStick]

LGTM!