refactor(repo): create repo package

- Refactor `installer.clone` into new `repo.Clone` package and method.

- Refactor `sheets.isGitDir` into `repo.GitDir`.

Both of these changes read better, and will facilitate cleaner
architecture when `--update` is implemented.

internal/installer/run.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/cheat/cheat/internal/config"
+	"github.com/cheat/cheat/internal/repo"
 )
 
 // Run runs the installer
@@ -45,7 +46,7 @@ func Run(configs string, confpath string) error {
 	if yes {
 		// clone the community cheatsheets
 		fmt.Printf("Cloning community cheatsheets to %s.\n", community)
-		if err := clone(community); err != nil {
+		if err := repo.Clone(community); err != nil {
 			return fmt.Errorf("failed to clone cheatsheets: %v", err)
 		}