Refactor LFS SSH and internal routers

[wxiaoguang]

My Gitea instance keeps reporting a lot of errors like "LFS SSH transfer connection denied, pure SSH protocol is disabled"

When I started debugging the problem, I found more problems to fix. This PR addresses most of them:

1. unnecessary server side error logs (change fail() to not log them)
2. the broken user2/lfs.git (added comments)
3. migratePushMirrors could fail when a repository doesn't exist (ignore them)
4. "Authorization" (internal&lfs) header conflicts, remove the tricky "swapAuth" and use "X-Gitea-Internal-Auth"
5. internal token comparing is not constant time (use ConstantTimeCompare, it wasn't a serous problem because in a real world it's nearly impossible to timing-attack the token, but good to fix and backport)
6. duplicate routers (introduce AddOwnerRepoGitLFSRoutes)
7. "internal (private)" routes shouldn't use web context, they should use private context
8. incorrect "path" usages (use "filepath")
9. incorrect mocked route point handling (need to check func nil correctly)

By the way, split some tests from "git general tests" to "git misc tests" (to keep "git_general_test.go" simple)

Then I tried to write a test for "LFS SSH Transfer" but I guess I haven't gotten the correct result. So the code is kept there (tests/integration/git_lfs_ssh_test.go) and a FIXME explains the details. Indeed it's worth to take a deep look (in another PR)

[lunny]

cmd/web.go:231:func1() [F] PANIC: runtime error: invalid memory address or nil pointer dereference
/Users/xxxxx/data/Projects/gitea/gitea/modules/web/handler.go:133 (0x103bbca54)
	toHandlerProvider.wrapHandlerProvider[...].func2: h := hp(next) // this handle could be dynamically generated, so we can't use it for debug info
/Users/xxxxx/data/Projects/gitea/gitea/modules/web/route.go:117 (0x103bbd6ff)
	(*Router).Methods: r.chiRouter.With(middlewares...).Method(methods, fullPattern, handlerFunc)
/Users/xxxxx/data/Projects/gitea/gitea/modules/web/route.go:150 (0x103bbdc23)
	(*Router).Post: r.Methods("POST", pattern, h...)
/Users/xxxxx/data/Projects/gitea/gitea/routers/common/lfs.go:16 (0x104795427)
	Routes.func1.AddOwnerRepoGitLFSRoutes.3: m.Post("/objects/batch", lfs.CheckAcceptMediaType, lfs.BatchHandler)
/Users/xxxxx/data/Projects/gitea/gitea/modules/web/route.go:68 (0x103bbcedb)
	(*Router).Group: fn()
/Users/xxxxx/data/Projects/gitea/gitea/routers/common/lfs.go:15 (0x10479539f)
	AddOwnerRepoGitLFSRoutes: m.Group("/{username}/{reponame}/info/lfs", func() {

[wxiaoguang]

Fixed in 9a876b7, Golang has some strange rules for "nil" comparing.

[lunny]

Can we backport this to v1.22?

[wxiaoguang]

-> Refactor LFS SSH and internal routers (#32473) #32479

[wxiaoguang]

Next: Fix LFS route mock and realm #32488