Expose {{.SelectedCommitRange}} to custom commands

It has fields .To and .From (the hashes of the last and the first selected
commits, respectively), and it is useful for creating git commands that act on a
range of commits.

pkg/integration/tests/custom_commands/selected_commit_range.go

@@ -0,0 +1,41 @@
+package custom_commands
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var SelectedCommitRange = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Use the {{ .SelectedCommitRange }} template variable",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupRepo: func(shell *Shell) {
+		shell.CreateNCommits(3)
+	},
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().CustomCommands = []config.CustomCommand{
+			{
+				Key:     "X",
+				Context: "global",
+				Command: `git log --format="%s" {{.SelectedCommitRange.From}}^..{{.SelectedCommitRange.To}} > file.txt`,
+			},
+		}
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Commits().Focus().
+			Lines(
+				Contains("commit 03").IsSelected(),
+				Contains("commit 02"),
+				Contains("commit 01"),
+			)
+
+		t.GlobalPress("X")
+		t.FileSystem().FileContent("file.txt", Equals("commit 03\n"))
+
+		t.Views().Commits().Focus().
+			Press(keys.Universal.RangeSelectDown)
+
+		t.GlobalPress("X")
+		t.FileSystem().FileContent("file.txt", Equals("commit 03\ncommit 02\n"))
+	},
+})