Support range select for staging/discarding files

As part of this, you must now press enter on a merge conflict file
to focus the merge view; you can no longer press space and if you do
it will raise an error.

pkg/integration/tests/file/discard_various_changes.go

@@ -0,0 +1,70 @@
+package file
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var DiscardVariousChanges = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Discarding all possible permutations of changed files",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+	},
+	SetupRepo: func(shell *Shell) {
+		createAllPossiblePermutationsOfChangedFiles(shell)
+	},
+
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		type statusFile struct {
+			status string
+			label  string
+		}
+
+		discardOneByOne := func(files []statusFile) {
+			for _, file := range files {
+				t.Views().Files().
+					IsFocused().
+					SelectedLine(Contains(file.status + " " + file.label)).
+					Press(keys.Universal.Remove)
+
+				t.ExpectPopup().Menu().
+					Title(Equals("Discard changes")).
+					Select(Contains("Discard all changes")).
+					Confirm()
+			}
+		}
+
+		discardOneByOne([]statusFile{
+			{status: "UA", label: "added-them-changed-us.txt"},
+			{status: "AA", label: "both-added.txt"},
+			{status: "DD", label: "both-deleted.txt"},
+			{status: "UU", label: "both-modded.txt"},
+			{status: "AU", label: "changed-them-added-us.txt"},
+			{status: "UD", label: "deleted-them.txt"},
+			{status: "DU", label: "deleted-us.txt"},
+		})
+
+		t.ExpectPopup().Confirmation().
+			Title(Equals("Continue")).
+			Content(Contains("All merge conflicts resolved. Continue?")).
+			Cancel()
+
+		discardOneByOne([]statusFile{
+			{status: "AM", label: "added-changed.txt"},
+			{status: "MD", label: "change-delete.txt"},
+			{status: "D ", label: "delete-change.txt"},
+			{status: "D ", label: "deleted-staged.txt"},
+			{status: " D", label: "deleted.txt"},
+			{status: "MM", label: "double-modded.txt"},
+			{status: "M ", label: "modded-staged.txt"},
+			{status: " M", label: "modded.txt"},
+			{status: "A ", label: "new-staged.txt"},
+			{status: "??", label: "new.txt"},
+			// the menu title only includes the new file
+			{status: "R ", label: "renamed.txt → renamed2.txt"},
+		})
+
+		t.Views().Files().IsEmpty()
+	},
+})