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/commands/oscommands/cmd_obj_test.go

@@ -0,0 +1,33 @@
+package oscommands
+
+import (
+	"testing"
+)
+
+func TestCmdObjToString(t *testing.T) {
+	quote := func(s string) string {
+		return "\"" + s + "\""
+	}
+
+	scenarios := []struct {
+		cmdArgs  []string
+		expected string
+	}{
+		{
+			cmdArgs:  []string{"git", "push", "myfile.txt"},
+			expected: "git push myfile.txt",
+		},
+		{
+			cmdArgs:  []string{"git", "push", "my file.txt"},
+			expected: "git push \"my file.txt\"",
+		},
+	}
+
+	for _, scenario := range scenarios {
+		cmdObj := &CmdObj{args: scenario.cmdArgs}
+		actual := cmdObj.ToString()
+		if actual != scenario.expected {
+			t.Errorf("Expected %s, got %s", quote(scenario.expected), quote(actual))
+		}
+	}
+}