Add a test demonstrating the problem

When pressing `e` on line 5 in a diff of an older commit, we expect it to take
us to line 5 in that file. But we end up on line 2, because the file had further
changes both in newer commits, and in the unstaged changes of the working copy.

pkg/integration/tests/patch_building/edit_line_in_patch_building_panel.go

@@ -0,0 +1,50 @@
+package patch_building
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var EditLineInPatchBuildingPanel = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Edit a line in the patch building panel; make sure we end up on the right line",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+		config.GetUserConfig().OS.EditAtLine = "echo {{filename}}:{{line}} > edit-command"
+	},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("file.txt", "4\n5\n6\n")
+		shell.Commit("01")
+		shell.UpdateFileAndAdd("file.txt", "1\n2a\n2b\n3\n4\n5\n6\n")
+		shell.Commit("02")
+		shell.UpdateFile("file.txt", "1\n2\n3\n4\n5\n6\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().
+			Focus().
+			Lines(
+				Contains("02").IsSelected(),
+				Contains("01"),
+			).
+			Press(keys.Universal.NextItem).
+			PressEnter()
+
+		t.Views().CommitFiles().
+			IsFocused().
+			Lines(
+				Contains("A file.txt").IsSelected(),
+			).
+			PressEnter()
+
+		t.Views().PatchBuilding().
+			IsFocused().
+			Content(Contains("+4\n+5\n+6")).
+			NavigateToLine(Contains("+5")).
+			Press(keys.Universal.Edit)
+
+		/* EXPECTED:
+		t.FileSystem().FileContent("edit-command", Contains("file.txt:5\n"))
+		ACTUAL: */
+		t.FileSystem().FileContent("edit-command", Contains("file.txt:2\n"))
+	},
+})

pkg/integration/tests/test_list.go

@@ -257,6 +257,7 @@ var tests = []*components.IntegrationTest{
 	patch_building.Apply,
 	patch_building.ApplyInReverse,
 	patch_building.ApplyInReverseWithConflict,
+	patch_building.EditLineInPatchBuildingPanel,
 	patch_building.MoveRangeToIndex,
 	patch_building.MoveToEarlierCommit,
 	patch_building.MoveToEarlierCommitFromAddedFile,