add a only_first_parent option to only consider commits on current branch

[eburghar]

Close #321

[codecov]

Codecov Report

Merging #324 (25684ef) into main (5b34ee4) will increase coverage by 0.29%.
Report is 3 commits behind head on main.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
+ Coverage   85.29%   85.58%   +0.29%     
==========================================
  Files          45       45              
  Lines        6283     6389     +106     
==========================================
+ Hits         5359     5468     +109     
+ Misses        924      921       -3     

Files	Coverage Δ
src/settings/mod.rs	75.48% <100.00%> (+0.19%)	⬆️
src/git/revspec.rs	95.45% <83.33%> (-0.51%)	⬇️

... and 10 files with indirect coverage changes

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