Add integrations tests from git cli

[sapk]

Extracted from #3152 for easy review process.

• Keep TestGit/SSH/LFS/Locks commented in the wait of Fix SSH auth lfs locks #3152
• Add big file test for covering Pushing large amount of data via git-lfs results in HTTP 413 after some time #2930
• Disable update of publickeyfile with internal server.

[codecov-io]

Codecov Report

Merging #3377 into master will increase coverage by 0.58%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master   #3377      +/-   ##
=========================================
+ Coverage   35.01%   35.6%   +0.58%     
=========================================
  Files         281     281              
  Lines       40560   40571      +11     
=========================================
+ Hits        14204   14447     +243     
+ Misses      24255   23985     -270     
- Partials     2101    2139      +38

Impacted Files	Coverage Δ
models/ssh_key.go	38.46% <50%> (-0.74%)	⬇️
models/repo_list.go	65.62% <0%> (-1.57%)	⬇️
modules/setting/setting.go	46.42% <0%> (-1.03%)	⬇️
models/repo.go	43.25% <0%> (-0.13%)	⬇️
models/repo_indexer.go	46.63% <0%> (ø)	⬆️
modules/lfs/server.go	37.66% <0%> (+2.65%)	⬆️
modules/lfs/locks.go	55.72% <0%> (+3.64%)	⬆️
routers/init.go	80% <0%> (+7.27%)	⬆️
routers/private/internal.go	59.09% <0%> (+13.63%)	⬆️
... and 3 more

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[lafriks]

Remove debug but otherwise LGTM

[sapk]

@lafriks done. I don't know why it fix the blocking situation in previous commit. (maybe related to drone ?)

[strk]

trusted LGTM although the code is very hard to review, lots of things removed!

[sapk]

@strk I refactor some code to be in some function like generateCommitWithNewData, commitAndPush, lockTest and that why I wanted to extract it from #3152 to "ease" the review.
This mostly duplicate existing http git clone to also do ssh tests.