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)

modules/git/submodule.go

@@ -7,7 +7,6 @@ import (
 	"bufio"
 	"context"
 	"fmt"
-	"io"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
 	"code.gitea.io/gitea/modules/log"
@@ -21,10 +20,10 @@ type TemplateSubmoduleCommit struct {
 // GetTemplateSubmoduleCommits returns a list of submodules paths and their commits from a repository
 // This function is only for generating new repos based on existing template, the template couldn't be too large.
 func GetTemplateSubmoduleCommits(ctx context.Context, repoPath string) (submoduleCommits []TemplateSubmoduleCommit, _ error) {
-	var stdoutReader io.ReadCloser
-	err := gitcmd.NewCommand("ls-tree", "-r", "--", "HEAD").
-		WithDir(repoPath).
-		WithStdoutReader(&stdoutReader).
+	cmd := gitcmd.NewCommand("ls-tree", "-r", "--", "HEAD")
+	stdoutReader, stdoutReaderClose := cmd.MakeStdoutPipe()
+	defer stdoutReaderClose()
+	err := cmd.WithDir(repoPath).
 		WithPipelineFunc(func(ctx gitcmd.Context) error {
 			scanner := bufio.NewScanner(stdoutReader)
 			for scanner.Scan() {