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/wiki/wiki.go

@@ -170,7 +170,7 @@ func updateWikiPage(ctx context.Context, doer *user_model.User, repo *repo_model
 
 	// FIXME: The wiki doesn't have lfs support at present - if this changes need to check attributes here
 
-	objectHash, err := gitRepo.HashObject(strings.NewReader(content))
+	objectHash, err := gitRepo.HashObjectBytes([]byte(content))
 	if err != nil {
 		log.Error("HashObject failed: %v", err)
 		return err