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/repo_stats.go

@@ -6,7 +6,6 @@ package git
 import (
 	"bufio"
 	"fmt"
-	"io"
 	"sort"
 	"strconv"
 	"strings"
@@ -62,10 +61,10 @@ func (repo *Repository) GetCodeActivityStats(fromTime time.Time, branch string)
 		gitCmd.AddArguments("--first-parent").AddDynamicArguments(branch)
 	}
 
-	var stdoutReader io.ReadCloser
+	stdoutReader, stdoutReaderClose := gitCmd.MakeStdoutPipe()
+	defer stdoutReaderClose()
 	err = gitCmd.
 		WithDir(repo.Path).
-		WithStdoutReader(&stdoutReader).
 		WithPipelineFunc(func(ctx gitcmd.Context) error {
 			scanner := bufio.NewScanner(stdoutReader)
 			scanner.Split(bufio.ScanLines)
@@ -117,7 +116,6 @@ func (repo *Repository) GetCodeActivityStats(fromTime time.Time, branch string)
 				}
 			}
 			if err = scanner.Err(); err != nil {
-				_ = stdoutReader.Close()
 				return fmt.Errorf("GetCodeActivityStats scan: %w", err)
 			}
 			a := make([]*CodeActivityAuthor, 0, len(authors))
@@ -131,7 +129,6 @@ func (repo *Repository) GetCodeActivityStats(fromTime time.Time, branch string)
 			stats.AuthorCount = int64(len(authors))
 			stats.ChangedFiles = int64(len(files))
 			stats.Authors = a
-			_ = stdoutReader.Close()
 			return nil
 		}).
 		RunWithStderr(repo.Ctx)