Some refactor on git diff and ignore getting commit information failed on migrating pull request review comments

[lunny]

When migrating a repository from github with pull requests, if the pull request review comments refs an outdate commit which is not yet on this repository any more. We should ignore that commits.

This pull request also moved some git diff functions from package gitdiff to git.

[codecov-io]

Codecov Report

Merging #9996 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9996      +/-   ##
==========================================
+ Coverage   42.26%   42.26%   +<.01%     
==========================================
  Files         611      611              
  Lines       80391    80391              
==========================================
+ Hits        33976    33978       +2     
+ Misses      42237    42235       -2     
  Partials     4178     4178

Impacted Files	Coverage Δ
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
models/repo.go	49.66% <0%> (-0.14%)	⬇️
services/pull/patch.go	67.92% <0%> (ø)	⬆️
modules/queue/workerpool.go	42.48% <0%> (+1.28%)	⬆️
modules/process/manager.go	78.31% <0%> (+3.61%)	⬆️

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[6543]

Beside modules/migrations/gitea.go it is a refactor

[zeripath]

@lunny it's hard to see what you've fixed and what's been moved - I wonder could you split the commits in two - one doing the move and one doing the fix?

[guillep2k]

Just a couple of nits.