Fixed bug where async_alert() overwrites readline's incremental and non-incremental search prompts.

[kmvanbrunt]

Fixes #1260

[codecov]

Codecov Report

Attention: Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.

Project coverage is 98.34%. Comparing base (ff46021) to head (3782243).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd2/rl_utils.py	8.33%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1330      +/-   ##
==========================================
+ Coverage   97.72%   98.34%   +0.62%     
==========================================
  Files          22       22              
  Lines        5753     5806      +53     
==========================================
+ Hits         5622     5710      +88     
+ Misses        131       96      -35     

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