Refactor git command stdio pipe (#36422)

Most potential deadlock problems should have been fixed, and new code is
unlikely to cause new problems with the new design.

Also raise the minimum Git version required to 2.6.0 (released in 2015)

services/mirror/mirror_push.go

@@ -192,7 +192,9 @@ func pushAllLFSObjects(ctx context.Context, gitRepo *git.Repository, lfsClient l
 
 	pointerChan := make(chan lfs.PointerBlob)
 	errChan := make(chan error, 1)
-	go lfs.SearchPointerBlobs(ctx, gitRepo, pointerChan, errChan)
+	go func() {
+		errChan <- lfs.SearchPointerBlobs(ctx, gitRepo, pointerChan)
+	}()
 
 	uploadObjects := func(pointers []lfs.Pointer) error {
 		err := lfsClient.Upload(ctx, pointers, func(p lfs.Pointer, objectError error) (io.ReadCloser, error) {
@@ -242,13 +244,12 @@ func pushAllLFSObjects(ctx context.Context, gitRepo *git.Repository, lfsClient l
 		}
 	}
 
-	err, has := <-errChan
-	if has {
+	err := <-errChan
+	if err != nil {
 		log.Error("Error enumerating LFS objects for repository: %v", err)
-		return err
 	}
 
-	return nil
+	return err
 }
 
 func syncPushMirrorWithSyncOnCommit(ctx context.Context, repoID int64) {