Fix #2830 - subgroups incorrectly shows as commands in help

[Johan Stenberg (ex-msft) (johanste)]

Fixed regression introduced by the performance work that made the help subsystem confused about commands vs. command groups (we now replace command groups we know won't match the arguments passed in with a dummy command).

Fixes #2830

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General Guidelines

• [n/a] The PR has modified HISTORY.rst with an appropriate description of the change (see Modifying change log).

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• [n/a ] Each command and parameter has a meaningful description.
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[Codecov (codecov-io)]

Codecov Report

Merging #2838 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2838      +/-   ##
==========================================
+ Coverage   62.88%   62.88%   +<.01%     
==========================================
  Files         464      464              
  Lines       25901    25901              
  Branches     3945     3944       -1     
==========================================
+ Hits        16287    16288       +1     
  Misses       8594     8594              
+ Partials     1020     1019       -1

Impacted Files	Coverage Δ
src/azure-cli-core/azure/cli/core/application.py	89.32% <100%> (+0.48%)	⬆️
src/azure-cli-core/azure/cli/core/parser.py	81.37% <100%> (ø)	⬆️

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