revert: revlist: do not revisit already visited ancestors

[erizocosmico]

Looks like the assumption for this optimization was not correct. I don't know why yet but empirically, revlist with this optimisation is different to revlist without it at least on https://github.com/thrasnic/mongoose.

Revlist with optimisation: 29050 objects
Without optimisation: 33916 objects

[codecov]

Codecov Report

Merging #584 into master will decrease coverage by 0.59%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master     #584     +/-   ##
=========================================
- Coverage   78.34%   77.74%   -0.6%     
=========================================
  Files         130      130             
  Lines       10097    10092      -5     
=========================================
- Hits         7910     7846     -64     
- Misses       1342     1414     +72     
+ Partials      845      832     -13

Impacted Files	Coverage Δ
plumbing/revlist/revlist.go	72.94% <66.66%> (-1.51%)	⬇️
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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