packp: fixed encoding when HEAD is not a valid ref

[mcarmonaa]

It solves issue #152

[codecov]

Codecov Report

Merging #552 into master will decrease coverage by 0.58%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #552      +/-   ##
=========================================
- Coverage   78.08%   77.5%   -0.59%     
=========================================
  Files         129     129              
  Lines        9798    9807       +9     
=========================================
- Hits         7651    7601      -50     
- Misses       1316    1388      +72     
+ Partials      831     818      -13

Impacted Files	Coverage Δ
plumbing/protocol/packp/advrefs_encode.go	87.34% <100%> (+1.62%)	⬆️
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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[mcuadros]

Awesome!

[ajnavarro]

good @mcarmonaa !!