[WIP] pkg/gui: Allow user to select remote and branch when creating a PR

When creating a PR against a selected branch (via O = "create pull request
options"), the user will first be asked to select a remote (if there is more
than one). After that, the suggestion area is populated with all remote branches
at that origin - instead of all local ones. After all, creating a PR against a
branch that doesn't exist on the remote won't work.

Please note that the "PR is not filed against 'origin' remote" use cases (e.g. when
contributing via a fork that is 'origin' to a GitHub project that is
'upstream'), is not working correctly yet.

Fixes #1826.

pkg/gui/controllers/branches_controller.go

@@ -728,13 +728,26 @@ func (self *BranchesController) createPullRequestMenu(selectedBranch *models.Bra
 				},
 			},
 			{
+				// TODO: Replace with "Select remote and branch"?
 				LabelColumns: fromToLabelColumns(branch.Name, self.c.Tr.SelectBranch),
 				OnPress: func() error {
+					if !branch.IsTrackingRemote() {
+						return errors.New(self.c.Tr.PullRequestNoUpstream)
+					}
+
+					if len(self.c.Model().Remotes) == 1 {
+						toRemote := self.c.Model().Remotes[0].Name
+						self.c.Log.Debugf("PR will target the only existing remote '%s'", toRemote)
+						return self.promptForTargetBranchNameAndCreatePullRequest(branch, toRemote)
+					}
+
 					self.c.Prompt(types.PromptOpts{
-						Title:               branch.Name + " →",
-						FindSuggestionsFunc: self.c.Helpers().Suggestions.GetRemoteBranchesSuggestionsFunc("/"),
-						HandleConfirm: func(targetBranchName string) error {
-							return self.createPullRequest(branch.Name, targetBranchName)
+						Title:               "Select Target Remote",
+						FindSuggestionsFunc: self.c.Helpers().Suggestions.GetRemoteSuggestionsFunc(),
+						HandleConfirm: func(toRemote string) error {
+							self.c.Log.Debugf("PR will target remote '%s'", toRemote)
+
+							return self.promptForTargetBranchNameAndCreatePullRequest(branch, toRemote)
 						},
 					})
 
@@ -764,6 +777,19 @@ func (self *BranchesController) createPullRequestMenu(selectedBranch *models.Bra
 	return self.c.Menu(types.CreateMenuOptions{Title: fmt.Sprint(self.c.Tr.CreatePullRequestOptions), Items: menuItems})
 }
 
+func (self *BranchesController) promptForTargetBranchNameAndCreatePullRequest(fromBranch *models.Branch, toRemote string) error {
+	self.c.Prompt(types.PromptOpts{
+		Title:               fmt.Sprintf("%s → %s/", fromBranch.UpstreamBranch, toRemote),
+		FindSuggestionsFunc: self.c.Helpers().Suggestions.GetRemoteBranchesForRemoteSuggestionsFunc(toRemote),
+		HandleConfirm: func(toBranch string) error {
+			self.c.Log.Debugf("PR will target branch '%s' on remote '%s'", toBranch, toRemote)
+			return self.createPullRequest(fromBranch.UpstreamBranch, toBranch)
+		},
+	})
+
+	return nil
+}
+
 func (self *BranchesController) createPullRequest(from string, to string) error {
 	url, err := self.c.Helpers().Host.GetPullRequestURL(from, to)
 	if err != nil {

pkg/gui/controllers/helpers/suggestions_helper.go

@@ -162,10 +162,38 @@ func (self *SuggestionsHelper) getRemoteBranchNames(separator string) []string {
 	})
 }
 
+func (self *SuggestionsHelper) getRemoteBranchNamesForRemote(remoteName string) []string {
+	for _, remote := range self.c.Model().Remotes {
+		if remote.Name == remoteName {
+			return lo.Map(remote.Branches, func(branch *models.RemoteBranch, _ int) string {
+				return branch.Name
+			})
+		}
+	}
+
+	return nil
+}
+
+func (self *SuggestionsHelper) getRemoteBranchNamesWithoutRemotePrefix() []string {
+	return lo.FlatMap(self.c.Model().Remotes, func(remote *models.Remote, _ int) []string {
+		return lo.Map(remote.Branches, func(branch *models.RemoteBranch, _ int) string {
+			return branch.Name
+		})
+	})
+}
+
 func (self *SuggestionsHelper) GetRemoteBranchesSuggestionsFunc(separator string) func(string) []*types.Suggestion {
 	return FilterFunc(self.getRemoteBranchNames(separator), self.c.UserConfig().Gui.UseFuzzySearch())
 }
 
+func (self *SuggestionsHelper) GetRemoteBranchesForRemoteSuggestionsFunc(remoteName string) func(string) []*types.Suggestion {
+	return FilterFunc(self.getRemoteBranchNamesForRemote(remoteName), self.c.UserConfig().Gui.UseFuzzySearch())
+}
+
+func (self *SuggestionsHelper) GetRemoteBranchesWithoutRemotePrefixSuggestionsFunc() func(string) []*types.Suggestion {
+	return FilterFunc(self.getRemoteBranchNamesWithoutRemotePrefix(), self.c.UserConfig().Gui.UseFuzzySearch())
+}
+
 func (self *SuggestionsHelper) getTagNames() []string {
 	return lo.Map(self.c.Model().Tags, func(tag *models.Tag, _ int) string {
 		return tag.Name