Add shell completion with cobra

Allow automatic generation for shell completion scripts
with the internal cobra functions (requires v1.0.0+).

This should replace the handwritten completion scripts
and even adds support for fish. With this approach it is
less likley that completions and code are out of sync.

We can now create the scripts with
- podman completion bash
- podman completion zsh
- podman completion fish

To test the completion run:
source <(podman completion bash)

The same works for podman-remote and podman --remote and
it will complete your remote containers/images with
the correct endpoints values from --url/--connection.

The completion logic is written in go and provided by the
cobra library. The completion functions lives in
`cmd/podman/completion/completion.go`.

The unit test at cmd/podman/shell_completion_test.go checks
if each command and flag has an autocompletion function set.
This prevents that commands and flags have no shell completion set.

This commit does not replace the current autocompletion scripts.

Closes #6440

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

cmd/podman/diff.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 
+	"github.com/containers/podman/v2/cmd/podman/common"
 	"github.com/containers/podman/v2/cmd/podman/containers"
 	"github.com/containers/podman/v2/cmd/podman/images"
 	"github.com/containers/podman/v2/cmd/podman/registry"
@@ -17,11 +18,12 @@ var (
 	// Command: podman _diff_ Object_ID
 	diffDescription = `Displays changes on a container or image's filesystem.  The container or image will be compared to its parent layer.`
 	diffCmd         = &cobra.Command{
-		Use:   "diff [options] {CONTAINER_ID | IMAGE_ID}",
-		Args:  validate.IDOrLatestArgs,
-		Short: "Display the changes to the object's file system",
-		Long:  diffDescription,
-		RunE:  diff,
+		Use:               "diff [options] {CONTAINER_ID | IMAGE_ID}",
+		Args:              validate.IDOrLatestArgs,
+		Short:             "Display the changes to the object's file system",
+		Long:              diffDescription,
+		RunE:              diff,
+		ValidArgsFunction: common.AutocompleteContainersAndImages,
 		Example: `podman diff imageID
   podman diff ctrID
   podman diff --format json redis:alpine`,
@@ -38,7 +40,11 @@ func init() {
 	flags := diffCmd.Flags()
 	flags.BoolVar(&diffOpts.Archive, "archive", true, "Save the diff as a tar archive")
 	_ = flags.MarkHidden("archive")
-	flags.StringVar(&diffOpts.Format, "format", "", "Change the output format")
+
+	formatFlagName := "format"
+	flags.StringVar(&diffOpts.Format, formatFlagName, "", "Change the output format")
+	_ = diffCmd.RegisterFlagCompletionFunc(formatFlagName, common.AutocompleteJSONFormat)
+
 	validate.AddLatestFlag(diffCmd, &diffOpts.Latest)
 }