refactor!: remove go-git backend (#39487)

Removes the go-git backend so every build uses the git CLI backend. Its
Windows performance advantage is gone, it lacks SHA-256 support, and it
breaks repositories on Windows.

The performance changes moved to
https://github.com/go-gitea/gitea/pull/39526.

Fixes https://github.com/go-gitea/gitea/issues/38359
Fixes https://github.com/go-gitea/gitea/issues/34694

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/renderhelper/commit_checker.go

@@ -50,7 +50,7 @@ func (c *commitChecker) IsCommitIDExisting(commitID string) bool {
 		c.gitRepo, c.gitRepoCloser = r, closer
 	}
 
-	exist = c.gitRepo.IsReferenceExist(c.ctx, commitID) // Don't use IsObjectExist since it doesn't support short hashes with gogit edition.
+	exist = c.gitRepo.IsReferenceExist(c.ctx, commitID)
 	c.commitCache[commitID] = exist
 	return exist
 }

models/unittest/fscopy.go

@@ -60,7 +60,6 @@ func SyncDirs(srcPath, destPath string) error {
 	}
 
 	// the keep file is used to keep the directory in a git repository, it doesn't need to be synced
-	// and go-git doesn't work with the ".keep" file (it would report errors like "ref is empty")
 	const keepFile = ".keep"
 
 	// find and delete all untracked files