Use StdEncoding for git hash directory name

[nkubala]

Fixes: #6025

Description

base64.UrlEncoding uses the "alternative alphabet" from RFC 4648, which supports the - characters. When this character is the leading character of a computed hash, using this as an argument to any git command causes an error because it's misinterpreted as a command switch.

base64.StdEncoding does not support the - character in its alphabet, so we should be able to use this encoder as a drop-in replacement with no issues.

[codecov]

Codecov Report

Merging #6071 (5e45356) into master (3a531d6) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6071      +/-   ##
==========================================
- Coverage   70.22%   70.21%   -0.02%     
==========================================
  Files         474      474              
  Lines       18165    18165              
==========================================
- Hits        12757    12755       -2     
- Misses       4473     4474       +1     
- Partials      935      936       +1     

Impacted Files	Coverage Δ
pkg/skaffold/git/gitutil.go	65.43% <100.00%> (ø)
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/skaffold/docker/parse.go	87.14% <0.00%> (+0.95%)	⬆️

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[MarlonGamez]

lgtm !