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/pull/temp_repo.go

@@ -5,11 +5,11 @@
 package pull
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
 
 	git_model "code.gitea.io/gitea/models/git"
 	issues_model "code.gitea.io/gitea/models/issues"
@@ -32,7 +32,7 @@ type prTmpRepoContext struct {
 	context.Context
 	tmpBasePath string
 	pr          *issues_model.PullRequest
-	outbuf      *strings.Builder // we keep these around to help reduce needless buffer recreation, any use should be preceded by a Reset and preferably after use
+	outbuf      *bytes.Buffer // we keep these around to help reduce needless buffer recreation, any use should be preceded by a Reset and preferably after use
 }
 
 // PrepareGitCmd prepares a git command with the correct directory, environment, and output buffers
@@ -40,7 +40,7 @@ type prTmpRepoContext struct {
 // Do NOT use it with gitcmd.RunStd*() functions, otherwise it will panic
 func (ctx *prTmpRepoContext) PrepareGitCmd(cmd *gitcmd.Command) *gitcmd.Command {
 	ctx.outbuf.Reset()
-	return cmd.WithDir(ctx.tmpBasePath).WithStdout(ctx.outbuf)
+	return cmd.WithDir(ctx.tmpBasePath).WithStdoutBuffer(ctx.outbuf)
 }
 
 // createTemporaryRepoForPR creates a temporary repo with "base" for pr.BaseBranch and "tracking" for  pr.HeadBranch
@@ -82,7 +82,7 @@ func createTemporaryRepoForPR(ctx context.Context, pr *issues_model.PullRequest)
 		Context:     ctx,
 		tmpBasePath: tmpBasePath,
 		pr:          pr,
-		outbuf:      &strings.Builder{},
+		outbuf:      &bytes.Buffer{},
 	}
 
 	baseRepoPath := pr.BaseRepo.RepoPath()