git/libgit2: assert proper test of default branch

If there is no configuration set for `init.defaultBranch`, it does not
return an error but an empty string. We now take this into account so
we do not overwrite the default, and make the default `master` to match
with libgit2 defaults.

In addition, some comments have been added to not get confused about
what commits we are checking against.

Signed-off-by: Hidde Beydals <hello@hidde.co>

pkg/git/libgit2/checkout_test.go

@@ -51,8 +51,8 @@ func TestCheckoutBranch_Checkout(t *testing.T) {
 
 	// ignores the error here because it can be defaulted
 	// https://github.blog/2020-07-27-highlights-from-git-2-28/#introducing-init-defaultbranch
-	defaultBranch := "main"
-	if v, err := cfg.LookupString("init.defaultBranch"); err != nil {
+	defaultBranch := "master"
+	if v, err := cfg.LookupString("init.defaultBranch"); err != nil && v != "" {
 		defaultBranch = v
 	}
 
@@ -61,10 +61,12 @@ func TestCheckoutBranch_Checkout(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// Branch off on first commit
 	if err = createBranch(repo, "test", nil); err != nil {
 		t.Fatal(err)
 	}
 
+	// Create second commit on default branch
 	secondCommit, err := commitFile(repo, "branch", "second", time.Now())
 	if err != nil {
 		t.Fatal(err)