Write unit tests with the help of afero

Afero is a package that lets you mock out a filesystem with an in-memory filesystem.
It allows us to easily create the files required for a given test without worrying about
a cleanup step or different tests tripping on eachother when run in parallel.

Later on I'll standardise on using afero over the vanilla os package

pkg/common/common.go

@@ -4,6 +4,7 @@ import (
 	"github.com/jesseduffield/lazygit/pkg/config"
 	"github.com/jesseduffield/lazygit/pkg/i18n"
 	"github.com/sirupsen/logrus"
+	"github.com/spf13/afero"
 )
 
 // Commonly used things wrapped into one struct for convenience when passing it around
@@ -12,4 +13,7 @@ type Common struct {
 	Tr         *i18n.TranslationSet
 	UserConfig *config.UserConfig
 	Debug      bool
+	// for interacting with the filesystem. We use afero rather than the default
+	// `os` package for the sake of mocking the filesystem in tests
+	Fs afero.Fs
 }