Provide arguments to NewGitServer and NewGitProtocol as Opts structs

[lhchavez]

There are now way too many arguments for these functions, so let's do a
small API-breaking change to make it easier to extend. #major

[codecov]

Codecov Report

Merging #13 (d2c1b6b) into main (81c647d) will increase coverage by 1.37%.
The diff coverage is 67.88%.

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   53.10%   54.47%   +1.37%     
==========================================
  Files           7        8       +1     
  Lines        2000     2135     +135     
==========================================
+ Hits         1062     1163     +101     
- Misses        730      746      +16     
- Partials      208      226      +18     

Impacted Files	Coverage Δ
commits.go	58.07% <40.00%> (+0.03%)	⬆️
browser.go	52.29% <48.14%> (-0.13%)	⬇️
keyedpool.go	78.12% <78.12%> (ø)
protocol.go	62.78% <78.94%> (-0.21%)	⬇️
server.go	21.37% <81.81%> (-0.16%)	⬇️
packfile.go	55.12% <100.00%> (ø)

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