fix up tests

This fixes up some git and oscommand tests, and pulls some tests into commit_list_builder_test.go

I've also made the NewDummyBlah functions public so that I didn't need to duplicate them across packages
I've also given OSCommand a SetCommand() method for setting the command on the struct
I've also created a file utils.go in the test package for creating convient 'CommandSwapper's, which
basically enable you to assert a sequence of commands on the command line, and swap each one out for
a different one to actually be executed

pkg/test/utils.go

@@ -0,0 +1,45 @@
+package test
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/mgutz/str"
+	"github.com/stretchr/testify/assert"
+)
+
+// CommandSwapper takes a command, verifies that it is what it's expected to be
+// and then returns a replacement command that will actually be called by the os
+type CommandSwapper struct {
+	Expect  string
+	Replace string
+}
+
+// SwapCommand verifies the command is what we expected, and swaps it out for a different command
+func (i *CommandSwapper) SwapCommand(t *testing.T, cmd string, args []string) *exec.Cmd {
+	splitCmd := str.ToArgv(i.Expect)
+	assert.EqualValues(t, splitCmd[0], cmd, fmt.Sprintf("received command: %s %s", cmd, strings.Join(args, " ")))
+	if len(splitCmd) > 1 {
+		assert.EqualValues(t, splitCmd[1:], args, fmt.Sprintf("received command: %s %s", cmd, strings.Join(args, " ")))
+	}
+
+	splitCmd = str.ToArgv(i.Replace)
+	return exec.Command(splitCmd[0], splitCmd[1:]...)
+}
+
+// CreateMockCommand creates a command function that will verify its receiving the right sequence of commands from lazygit
+func CreateMockCommand(t *testing.T, swappers []*CommandSwapper) func(cmd string, args ...string) *exec.Cmd {
+	commandIndex := 0
+
+	return func(cmd string, args ...string) *exec.Cmd {
+		var command *exec.Cmd
+		if commandIndex > len(swappers)-1 {
+			assert.Fail(t, fmt.Sprintf("too many commands run. This command was (%s %s)", cmd, strings.Join(args, " ")))
+		}
+		command = swappers[commandIndex].SwapCommand(t, cmd, args)
+		commandIndex++
+		return command
+	}
+}