mr: state that the MR ID parameter can be the branch name

The name of a MR's source branch can be used instead of the numeric
MR id parameter on the command line. Advertise this in the command
synopses.

The branch name is easier to remember, and plays better with standard
Git tooling than the opaque MR ids.

While at it, add square brackets to mark that the MR id parameter
is optional.  If it's left out, we use the MR associated with
the branch pointed to by HEAD.

Signed-off-by: Johannes Altmanninger <aclopte@gmail.com>

cmd/mr_approve.go

@@ -11,7 +11,7 @@ import (
 )
 
 var mrApproveCmd = &cobra.Command{
-	Use:     "approve [remote] <id>",
+	Use:     "approve [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Approve merge request",
 	Example: heredoc.Doc(`

cmd/mr_browse.go

@@ -12,7 +12,7 @@ import (
 )
 
 var mrBrowseCmd = &cobra.Command{
-	Use:              "browse [remote] <id>",
+	Use:              "browse [remote] [<MR id or branch>]",
 	Aliases:          []string{"b"},
 	Short:            "View merge request in a browser",
 	Example:          "lab mr browse origin",

cmd/mr_checkout.go

@@ -27,7 +27,7 @@ var (
 
 // listCmd represents the list command
 var checkoutCmd = &cobra.Command{
-	Use:   "checkout [remote] <id>",
+	Use:   "checkout [remote] [<MR id or branch>]",
 	Short: "Checkout an open merge request",
 	Args:  cobra.RangeArgs(1, 2),
 	Example: heredoc.Doc(`

cmd/mr_close.go

@@ -10,7 +10,7 @@ import (
 )
 
 var mrCloseCmd = &cobra.Command{
-	Use:              "close [remote] <id>",
+	Use:              "close [remote] [<MR id or branch>]",
 	Short:            "Close merge request",
 	Example:          "lab mr close origin 10",
 	PersistentPreRun: labPersistentPreRun,

cmd/mr_delete.go

@@ -9,7 +9,7 @@ import (
 )
 
 var mrDeleteCmd = &cobra.Command{
-	Use:     "delete [remote] <id>",
+	Use:     "delete [remote] [<MR id or branch>]",
 	Aliases: []string{"del"},
 	Short:   "Delete a merge request on GitLab",
 	Long: heredoc.Doc(`

cmd/mr_discussion.go

@@ -17,7 +17,7 @@ import (
 )
 
 var mrCreateDiscussionCmd = &cobra.Command{
-	Use:     "discussion [remote] <id>",
+	Use:     "discussion [remote] [<MR ID or branch>]",
 	Short:   "Start a discussion on an MR on GitLab",
 	Aliases: []string{"block", "thread"},
 	Example: heredoc.Doc(`

cmd/mr_merge.go

@@ -14,7 +14,7 @@ import (
 var mergeImmediate bool
 
 var mrMergeCmd = &cobra.Command{
-	Use:   "merge [remote] <id>",
+	Use:   "merge [remote] [<MR id or branch>]",
 	Short: "Merge an open merge request",
 	Long: heredoc.Doc(`
 		Merges an open merge request. If the pipeline in the project is

cmd/mr_rebase.go

@@ -8,7 +8,7 @@ import (
 )
 
 var mrRebaseCmd = &cobra.Command{
-	Use:              "rebase [remote] <id>",
+	Use:              "rebase [remote] [<MR id or branch>]",
 	Short:            "Rebase an open merge request",
 	Example:          "lab mr rebase upstream 20",
 	PersistentPreRun: labPersistentPreRun,

cmd/mr_reopen.go

@@ -10,7 +10,7 @@ import (
 )
 
 var mrReopenCmd = &cobra.Command{
-	Use:              "reopen [remote] <id>",
+	Use:              "reopen [remote] [<MR id or branch>]",
 	Short:            "Reopen a closed merge request",
 	Example:          "lab mr reopen upstream 20",
 	PersistentPreRun: labPersistentPreRun,

cmd/mr_show.go

@@ -21,7 +21,7 @@ var (
 )
 
 var mrShowCmd = &cobra.Command{
-	Use:        "show [remote] <id>",
+	Use:        "show [remote] [<MR id or branch>]",
 	Aliases:    []string{"get"},
 	ArgAliases: []string{"s"},
 	Short:      "Describe a merge request",

cmd/mr_subscribe.go

@@ -11,7 +11,7 @@ import (
 )
 
 var mrSubscribeCmd = &cobra.Command{
-	Use:     "subscribe [remote] <id>",
+	Use:     "subscribe [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Subscribe to merge request",
 	Example: heredoc.Doc(`

cmd/mr_thumb.go

@@ -18,7 +18,7 @@ var mrThumbCmd = &cobra.Command{
 }
 
 var mrThumbUpCmd = &cobra.Command{
-	Use:     "up [remote] <id>",
+	Use:     "up [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Thumb up merge request",
 	Example: heredoc.Doc(`
@@ -45,7 +45,7 @@ var mrThumbUpCmd = &cobra.Command{
 }
 
 var mrThumbDownCmd = &cobra.Command{
-	Use:     "down [remote] <id>",
+	Use:     "down [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Thumbs down merge request",
 	Example: heredoc.Doc(`

cmd/mr_unapprove.go

@@ -12,7 +12,7 @@ import (
 )
 
 var mrUnapproveCmd = &cobra.Command{
-	Use:     "unapprove [remote] <id>",
+	Use:     "unapprove [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Unapprove merge request",
 	Example: heredoc.Doc(`

cmd/mr_unsubscribe.go

@@ -11,7 +11,7 @@ import (
 )
 
 var mrUnsubscribeCmd = &cobra.Command{
-	Use:     "unsubscribe [remote] <id>",
+	Use:     "unsubscribe [remote] [<MR id or branch>]",
 	Aliases: []string{},
 	Short:   "Unubscribe from merge request",
 	Example: heredoc.Doc(`