[WIP] Add a benchmark for pull merging

[typeless]

This is going to to be used for measuring the speed improvement of the pull merging code.

[codecov-io]

Codecov Report

Merging #4908 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4908      +/-   ##
==========================================
- Coverage   37.56%   37.55%   -0.01%     
==========================================
  Files         321      321              
  Lines       47206    47206              
==========================================
- Hits        17732    17729       -3     
+ Misses      26933    26931       -2     
- Partials     2541     2546       +5

Impacted Files	Coverage Δ
models/repo_indexer.go	44.49% <0%> (-3.82%)	⬇️
routers/repo/view.go	46.01% <0%> (+1.22%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[lunny]

@typeless is this ready to review?

[typeless]

@lunny The repo tested in this PR is not big enough to confirm the effectiveness of #4921. I was considering what you suggested the other day using a Docker container for the prepared repo. But I have no a clear roadmap on how to do it (adding sane test fixtures to emulate a ready-for-merging-PRs repo is too hard).

[lunny]

@typeless why close this?

[typeless]

@lunny I think this PR is doing it the wrong way. It uses migrate to download a big (but not big enough) repo for benchmarking, which is very time-consuming. The ideal approach would be to prepare a test fixture which emulates the state of a ready-to-merge PR and probably a docker image containing a large repo as you suggested before.