fix(cli): remove confusion between {keep,ignore}-existing

[gregberge]

Closes #390

Summary

Test plan

[vercel]

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[codecov]

Codecov Report

Merging #413 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #413   +/-   ##
=======================================
  Coverage   85.55%   85.55%           
=======================================
  Files          30       30           
  Lines         526      526           
  Branches      143      143           
=======================================
  Hits          450      450           
  Misses         63       63           
  Partials       13       13           

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