remote: iterate over references only once

[erizocosmico]

In a push local references would be iterated one time per pushed reference, that gets super expensive as the number of references increases, since every time we iterate over all references, they're read from the disk. Instead of that, we calculate the refs ahead of time and send them down the line to all the functions that need them.

[codecov]

Codecov Report

Merging #580 into master will decrease coverage by 0.59%.
The diff coverage is 81.25%.

@@            Coverage Diff            @@
##           master     #580     +/-   ##
=========================================
- Coverage   78.04%   77.45%   -0.6%     
=========================================
  Files         129      129             
  Lines        9953     9963     +10     
=========================================
- Hits         7768     7717     -51     
- Misses       1340     1413     +73     
+ Partials      845      833     -12

Impacted Files	Coverage Δ
remote.go	74.58% <81.25%> (+0.13%)	⬆️
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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