Fix Stashing partial files for git version >= 2.35.0 (#3569)

- **PR Description**
Use git's `--staged` flag to stash staged changes if available (requires
git 2.35.0 or later), and fall back to our previous method if not. This
is a lot faster than our previous method, and it fixes two bugs, see
linked issues.

Fixes #3333 and #3563.

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

pkg/commands/git_commands/stash.go

@@ -121,9 +121,22 @@ func (self *StashCommands) StashUnstagedChanges(message string) error {
 	return nil
 }
 
-// SaveStagedChanges stashes only the currently staged changes. This takes a few steps
-// shoutouts to Joe on https://stackoverflow.com/questions/14759748/stashing-only-staged-changes-in-git-is-it-possible
+// SaveStagedChanges stashes only the currently staged changes.
 func (self *StashCommands) SaveStagedChanges(message string) error {
+	if self.version.IsAtLeast(2, 35, 0) {
+		return self.cmd.New(NewGitCmd("stash").Arg("push").Arg("--staged").Arg("-m", message).ToArgv()).Run()
+	}
+
+	// Git versions older than 2.35.0 don't support the --staged flag, so we
+	// need to fall back to a more complex solution.
+	// Shoutouts to Joe on https://stackoverflow.com/questions/14759748/stashing-only-staged-changes-in-git-is-it-possible
+	//
+	// Note that this method has a few bugs:
+	// - it fails when there are *only* staged changes
+	// - it fails when staged and unstaged changes within a single file are too close together
+	// We don't bother fixing these, because users can simply update git when
+	// they are affected by these issues.
+
 	// wrap in 'writing', which uses a mutex
 	if err := self.cmd.New(
 		NewGitCmd("stash").Arg("--keep-index").ToArgv(),

pkg/integration/tests/stash/stash_staged_partial_file.go

@@ -0,0 +1,70 @@
+package stash
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var StashStagedPartialFile = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Stash staged changes when a file is partially staged",
+	ExtraCmdArgs: []string{},
+	GitVersion:   AtLeast("git version 2.35.0"),
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("file-staged", "line1\nline2\nline3\nline4\n")
+		shell.Commit("initial commit")
+		shell.UpdateFile("file-staged", "line1\nline2 mod\nline3\nline4 mod\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			PressEnter()
+
+		t.Views().Staging().
+			Content(
+				Contains(" line1\n-line2\n+line2 mod\n line3\n-line4\n+line4 mod\n"),
+			).
+			PressPrimaryAction().
+			PressPrimaryAction().
+			Content(
+				Contains(" line1\n line2 mod\n line3\n-line4\n+line4 mod\n"),
+			).
+			PressEscape()
+
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Files.ViewStashOptions)
+
+		t.ExpectPopup().Menu().Title(Equals("Stash options")).Select(MatchesRegexp("Stash staged changes$")).Confirm()
+
+		t.ExpectPopup().Prompt().Title(Equals("Stash changes")).Type("my stashed file").Confirm()
+
+		t.Views().Stash().
+			Focus().
+			Lines(
+				Contains("my stashed file"),
+			).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("file-staged").IsSelected(),
+			)
+		t.Views().Main().
+			Content(
+				Contains(" line1\n-line2\n+line2 mod\n line3\n line4\n"),
+			)
+
+		t.Views().Files().
+			Lines(
+				Contains("file-staged"),
+			)
+
+		t.Views().Staging().
+			Content(
+				Contains(" line1\n line2\n line3\n-line4\n+line4 mod\n"),
+			)
+	},
+})

pkg/integration/tests/test_list.go

@@ -257,6 +257,7 @@ var tests = []*components.IntegrationTest{
 	stash.StashAndKeepIndex,
 	stash.StashIncludingUntrackedFiles,
 	stash.StashStaged,
+	stash.StashStagedPartialFile,
 	stash.StashUnstaged,
 	status.ClickRepoNameToOpenReposMenu,
 	status.ClickToFocus,