Suggest existing remote for non-tracking branch

Currently, when pushing or pulling a branch that has no tracking remote,
lazygit suggests the (hard-coded) remote named 'origin'. However, a
repository might not have a remote with this name, in which case the
suggestion makes no sense. This happens to me quite regularly when I
choose a more meaningful name than 'origin' for a remote.

This change keeps the current behavior by suggesting 'origin' when there
is either a remote with that name or no remote at all. However, when
'origin' does not exist, the name of the first remote is suggested.

Suggest existing remote for non-tracking branch

Currently, when pushing or pulling a branch that has no tracking remote,
lazygit suggests the (hard-coded) remote named 'origin'. However, a
repository might not have a remote with this name, in which case the
suggestion makes no sense. This happens to me quite regularly when I
choose a more meaningful name than 'origin' for a remote.

This change keeps the current behavior by suggesting 'origin' when there
is either a remote with that name or no remote at all. However, when
'origin' does not exist, the name of the first existing remote is
suggested.

pkg/gui/files_panel_test.go

@@ -0,0 +1,34 @@
+package gui
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetSuggestedRemote(t *testing.T) {
+	cases := []struct {
+		remotes  []*models.Remote
+		expected string
+	}{
+		{mkRemoteList(), "origin"},
+		{mkRemoteList("upstream", "origin", "foo"), "origin"},
+		{mkRemoteList("upstream", "foo", "bar"), "upstream"},
+	}
+
+	for _, c := range cases {
+		result := getSuggestedRemote(c.remotes)
+		assert.EqualValues(t, c.expected, result)
+	}
+}
+
+func mkRemoteList(names ...string) []*models.Remote {
+	var result []*models.Remote
+
+	for _, name := range names {
+		result = append(result, &models.Remote{Name: name})
+	}
+
+	return result
+}