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_compare.go

@@ -45,7 +45,7 @@ func (repo *Repository) GetDiffNumChangedFiles(base, head string, directComparis
 		AddDynamicArguments(base + separator + head).
 		AddArguments("--").
 		WithDir(repo.Path).
-		WithStdout(w).
+		WithStdoutCopy(w).
 		RunWithStderr(repo.Ctx); err != nil {
 		if strings.Contains(err.Stderr(), "no merge base") {
 			// git >= 2.28 now returns an error if base and head have become unrelated.
@@ -55,7 +55,7 @@ func (repo *Repository) GetDiffNumChangedFiles(base, head string, directComparis
 				AddDynamicArguments(base, head).
 				AddArguments("--").
 				WithDir(repo.Path).
-				WithStdout(w).
+				WithStdoutCopy(w).
 				RunWithStderr(repo.Ctx); err == nil {
 				return w.numLines, nil
 			}
@@ -71,7 +71,7 @@ var patchCommits = regexp.MustCompile(`^From\s(\w+)\s`)
 func (repo *Repository) GetDiff(compareArg string, w io.Writer) error {
 	return gitcmd.NewCommand("diff", "-p").AddDynamicArguments(compareArg).
 		WithDir(repo.Path).
-		WithStdout(w).
+		WithStdoutCopy(w).
 		Run(repo.Ctx)
 }
 
@@ -80,7 +80,7 @@ func (repo *Repository) GetDiffBinary(compareArg string, w io.Writer) error {
 	return gitcmd.NewCommand("diff", "-p", "--binary", "--histogram").
 		AddDynamicArguments(compareArg).
 		WithDir(repo.Path).
-		WithStdout(w).
+		WithStdoutCopy(w).
 		Run(repo.Ctx)
 }
 
@@ -88,7 +88,7 @@ func (repo *Repository) GetDiffBinary(compareArg string, w io.Writer) error {
 func (repo *Repository) GetPatch(compareArg string, w io.Writer) error {
 	return gitcmd.NewCommand("format-patch", "--binary", "--stdout").AddDynamicArguments(compareArg).
 		WithDir(repo.Path).
-		WithStdout(w).
+		WithStdoutCopy(w).
 		Run(repo.Ctx)
 }