Review GitModule.GetSelectedBranchFast

[drewnoakes]

Changes proposed in this pull request:

• Document what's going on.
• Do fewer string allocations.

What did I do to test the code and ensure quality:

• Manual testing

Has been tested on:

• GIT 2.16
• Windows 10

[codecov]

Codecov Report

Merging #4707 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #4707   +/-   ##
=======================================
  Coverage   28.23%   28.23%           
=======================================
  Files         511      511           
  Lines       41082    41082           
  Branches     5938     5938           
=======================================
  Hits        11598    11598           
  Misses      28966    28966           
  Partials      518      518

Impacted Files	Coverage Δ
GitCommands/Git/GitModule.cs	8.83% <0%> (-0.01%)	⬇️
GitUI/Plugin/LoadPlugins.cs	0% <0%> (ø)	⬆️
GitUI/GitUIExtensions.cs	0% <0%> (ø)	⬆️

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