Construct arg vector manually rather than parse string

By constructing an arg vector manually, we no longer need to quote arguments

Mandate that args must be passed when building a command

Now you need to provide an args array when building a command.
There are a handful of places where we need to deal with a string,
such as with user-defined custom commands, and for those we now require
that at the callsite they use str.ToArgv to do that. I don't want
to provide a method out of the box for it because I want to discourage its
use.

For some reason we were invoking a command through a shell when amending a
commit, and I don't believe we needed to do that as there was nothing user-
supplied about the command. So I've switched to using a regular command out-
side the shell there

pkg/integration/tests/custom_commands/complex_cmd_at_runtime.go

@@ -0,0 +1,35 @@
+package custom_commands
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var ComplexCmdAtRuntime = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Using a custom command provided at runtime to create a new file, via a shell command. We invoke custom commands through a shell already. This test proves that we can run a shell within a shell, which requires complex escaping.",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupRepo: func(shell *Shell) {
+		shell.EmptyCommit("blah")
+	},
+	SetupConfig: func(cfg *config.AppConfig) {},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsEmpty().
+			IsFocused().
+			Press(keys.Universal.ExecuteCustomCommand)
+
+		t.ExpectPopup().Prompt().
+			Title(Equals("Custom Command:")).
+			Type("sh -c \"touch file.txt\"").
+			Confirm()
+
+		t.GlobalPress(keys.Files.RefreshFiles)
+
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains("file.txt"),
+			)
+	},
+})