Refactor git command context & pipeline (#36406)

Less and simpler code, fewer bugs

modules/git/gitcmd/context.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitcmd
+
+import (
+	"context"
+)
+
+type Context interface {
+	context.Context
+
+	// CancelWithCause is a helper function to cancel the context with a specific error cause
+	// And it returns the same error for convenience, to break the PipelineFunc easily
+	CancelWithCause(err error) error
+
+	// In the future, this interface will be extended to support stdio pipe readers/writers
+}
+
+type cmdContext struct {
+	context.Context
+	cmd *Command
+}
+
+func (c *cmdContext) CancelWithCause(err error) error {
+	c.cmd.cmdCancel(err)
+	return err
+}