cmptimes: Code improvements to get_git_head()

[jwodder]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.04%. Comparing base (44c021e) to head (c820479).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
tools/cmptimes/src/main.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #219   +/-   ##
=======================================
  Coverage   74.03%   74.04%           
=======================================
  Files         409      409           
  Lines       22426    22423    -3     
=======================================
  Hits        16604    16604           
+ Misses       5822     5819    -3     

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