refactor: use WithRepo instead of WithDir for most git operations, clean up model migrations (#38555)

by the way, remove some unnecessary "models" imports from model
migration package, fix migration test model init bug
(modelmigration/migrationtest/tests.go)

modules/git/gitcmd/repo.go

@@ -25,7 +25,7 @@ type RepositoryFacade interface {
 }
 
 func (c *Command) WithRepo(repo RepositoryFacade) *Command {
-	c.opts.Dir = RepoLocalPath(repo)
+	c.gitDir = RepoLocalPath(repo)
 	return c
 }