Move HttpBackend function to Http to reduce function calls when git smart http requests

[lunny]

Before this change, we always create a closure function when response a smart git http request. This PR moved all codes to function Http to reduce the unnecessary closure function invoke.

[codecov-io]

Codecov Report

Merging #9057 into master will increase coverage by <.01%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master    #9057      +/-   ##
==========================================
+ Coverage   41.14%   41.14%   +<.01%     
==========================================
  Files         549      549              
  Lines       71506    71507       +1     
==========================================
+ Hits        29418    29423       +5     
  Misses      38365    38365              
+ Partials     3723     3719       -4

Impacted Files	Coverage Δ
routers/repo/http.go	35.38% <30.76%> (+0.15%)	⬆️
modules/task/migrate.go	25% <0%> (-3.95%)	⬇️
models/error.go	33.26% <0%> (-0.6%)	⬇️
models/repo.go	45.23% <0%> (+0.04%)	⬆️
modules/migrations/gitea.go	9.45% <0%> (+0.63%)	⬆️
modules/migrations/migrate.go	25.4% <0%> (+1.62%)	⬆️
models/pull_list.go	59.85% <0%> (+2.11%)	⬆️

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[zeripath]

As far as I was aware in go Closures aren't expensive to run - however this code doesn't need to be a closure (at least not any more - my suspicion is this is left over from some previous router structure).

This refactor makes the HTTP() function even longer and more complex. It's already too complex. We should be moving code out of HTTP() not in to it.

You could simply change the signature of the HTTPBackend to make it be a normal function instead.

[lunny]

@zeripath I moved them back to HTTP to prepare for next refactor PR. Currently we have two routers here, one is Macaron, another is custom regex routers. I think next refactor PR should just use Macaron routers.