Fix spelling Unstagged -> Unstaged

[blacksails]

This pull request fixes a spelling error.

The error is in the name of an exported variable and should therefore be considered backwards incompatible.

[codecov]

Codecov Report

Merging #615 into master will decrease coverage by 0.58%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #615      +/-   ##
=========================================
- Coverage   78.28%   77.7%   -0.59%     
=========================================
  Files         130     130              
  Lines       10160   10160              
=========================================
- Hits         7954    7895      -59     
- Misses       1351    1423      +72     
+ Partials      855     842      -13

Impacted Files	Coverage Δ
worktree.go	66.75% <50%> (ø)	⬆️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

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