fix: ensure .git repo exists

Part of coop-cloud/organising#247.

pkg/git/common.go

@@ -0,0 +1,14 @@
+package git
+
+import (
+	"fmt"
+	"os"
+)
+
+// EnsureGitRepo ensures a git repo .git folder exists
+func EnsureGitRepo(repoPath string) error {
+	if _, err := os.Stat(repoPath); os.IsNotExist(err) {
+		return fmt.Errorf("no .git directory in %s?", repoPath)
+	}
+	return nil
+}