dotgit: avoid duplicated references returned by Refs

[erizocosmico]

Refs could have duplicated references due to a race condition checking the mod time of packed-refs. Happened in https://travis-ci.org/src-d/borges/builds/263088460

ModTime returns the exact same time, but the content inside packed-refs is not the same.

[codecov]

Codecov Report

Merging #569 into master will decrease coverage by 0.58%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master     #569      +/-   ##
==========================================
- Coverage      78%   77.41%   -0.59%     
==========================================
  Files         129      129              
  Lines        9851     9855       +4     
==========================================
- Hits         7684     7629      -55     
- Misses       1328     1400      +72     
+ Partials      839      826      -13

Impacted Files	Coverage Δ
storage/filesystem/internal/dotgit/dotgit.go	72.03% <78.57%> (+0.43%)	⬆️
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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