When git version >= 2.18, git command could run with git wire protocol version 2 param if enabled

[lunny]

This PR will accelerate many git operations with remotes, especial migrations and mirrors.

[codecov-io]

Codecov Report

Merging #7047 into master will decrease coverage by 0.02%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master    #7047      +/-   ##
==========================================
- Coverage   41.46%   41.44%   -0.03%     
==========================================
  Files         442      442              
  Lines       59597    59604       +7     
==========================================
- Hits        24714    24704      -10     
- Misses      31654    31670      +16     
- Partials     3229     3230       +1

Impacted Files	Coverage Δ
modules/setting/git.go	52.17% <44.44%> (-10.33%)	⬇️
models/unit.go	62.16% <0%> (-5.41%)	⬇️
modules/log/file.go	75.52% <0%> (-2.1%)	⬇️
modules/log/event.go	64.46% <0%> (-1.53%)	⬇️
routers/repo/view.go	42.02% <0%> (-1.02%)	⬇️

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