Support suggestions generated from command in custom commands

This changes the interface a bit but it was only added earlier today so I doubt anybody is dependent on it yet.

I'm also updating the docs.

pkg/integration/tests/custom_commands/suggestions_command.go

@@ -0,0 +1,66 @@
+package custom_commands
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var SuggestionsCommand = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Using a custom command that uses a suggestions command in a prompt step",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupRepo: func(shell *Shell) {
+		shell.NewBranch("branch-one")
+		shell.EmptyCommit("blah")
+		shell.NewBranch("branch-two")
+		shell.EmptyCommit("blah")
+		shell.NewBranch("branch-three")
+		shell.EmptyCommit("blah")
+		shell.NewBranch("branch-four")
+		shell.EmptyCommit("blah")
+	},
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.UserConfig.CustomCommands = []config.CustomCommand{
+			{
+				Key:     "a",
+				Context: "localBranches",
+				Command: `git checkout {{.Form.Branch}}`,
+				Prompts: []config.CustomCommandPrompt{
+					{
+						Key:   "Branch",
+						Type:  "input",
+						Title: "Enter a branch name",
+						Suggestions: config.CustomCommandSuggestions{
+							Command: "git branch --format='%(refname:short)'",
+						},
+					},
+				},
+			},
+		}
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Focus().
+			Lines(
+				Contains("branch-four").IsSelected(),
+				Contains("branch-three"),
+				Contains("branch-two"),
+				Contains("branch-one"),
+			).
+			Press("a")
+
+		t.ExpectPopup().Prompt().
+			Title(Equals("Enter a branch name")).
+			Type("three").
+			SuggestionLines(Contains("branch-three")).
+			ConfirmFirstSuggestion()
+
+		t.Views().Branches().
+			Lines(
+				Contains("branch-three").IsSelected(),
+				Contains("branch-four"),
+				Contains("branch-two"),
+				Contains("branch-one"),
+			)
+	},
+})