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/gitcmd/error.go

@@ -76,3 +76,26 @@ func IsErrorCanceledOrKilled(err error) bool {
 	// TODO: in the future, we need to use unified error type from gitcmd.Run to check whether it is manually canceled
 	return errors.Is(err, context.Canceled) || IsErrorSignalKilled(err)
 }
+
+type pipelineError struct {
+	error
+}
+
+func (e pipelineError) Unwrap() error {
+	return e.error
+}
+
+func wrapPipelineError(err error) error {
+	if err == nil {
+		return nil
+	}
+	return pipelineError{err}
+}
+
+func ErrorAsPipeline(err error) error {
+	var pipelineErr pipelineError
+	if errors.As(err, &pipelineErr) {
+		return pipelineErr.error
+	}
+	return nil
+}