Avoid crash when the git command to obtain branches fails

This is probably not a good enough solution, because we only log the error (in
RefreshHelper.refreshBranches) and don't communicate it otherwise to users. This
is true for all errors that happen during refresh, though.

pkg/commands/git_commands/branch_loader.go

@@ -72,7 +72,10 @@ func (self *BranchLoader) Load(reflogCommits []*models.Commit,
 	onWorker func(func() error),
 	renderFunc func(),
 ) ([]*models.Branch, error) {
-	branches := self.obtainBranches()
+	branches, err := self.obtainBranches()
+	if err != nil {
+		return nil, err
+	}
 
 	if self.AppState.LocalBranchSortOrder == "recency" {
 		reflogBranches := self.obtainReflogBranches(reflogCommits)
@@ -232,16 +235,16 @@ func (self *BranchLoader) GetBaseBranch(branch *models.Branch, mainBranches *Mai
 	return split[0], nil
 }
 
-func (self *BranchLoader) obtainBranches() []*models.Branch {
+func (self *BranchLoader) obtainBranches() ([]*models.Branch, error) {
 	output, err := self.getRawBranches()
 	if err != nil {
-		panic(err)
+		return nil, err
 	}
 
 	trimmedOutput := strings.TrimSpace(output)
 	outputLines := strings.Split(trimmedOutput, "\n")
 
-	return lo.FilterMap(outputLines, func(line string, _ int) (*models.Branch, bool) {
+	branches := lo.FilterMap(outputLines, func(line string, _ int) (*models.Branch, bool) {
 		if line == "" {
 			return nil, false
 		}
@@ -257,6 +260,7 @@ func (self *BranchLoader) obtainBranches() []*models.Branch {
 		storeCommitDateAsRecency := self.AppState.LocalBranchSortOrder != "recency"
 		return obtainBranch(split, storeCommitDateAsRecency), true
 	})
+	return branches, nil
 }
 
 func (self *BranchLoader) getRawBranches() (string, error) {