Only verify the parent commit when it is going to be used

Moved the parent verification inside the branch that actually
depends on the parent. Without that it fails needlessly when
there is no parent yet.

Author: OskiKervinen-MF

lib/git-subrepo

@@ -701,18 +701,19 @@ subrepo:branch() {
 
   o "Subrepo parent: $subrepo_parent"
 
-  # Check if the subrepo parent is an ancestor of HEAD.
-  # For example rebasing the commit that touched the subrepo
-  # could make the commit no longer match.
-  local parent_is_ancestor=
-  git merge-base --is-ancestor "$subrepo_parent" HEAD && parent_is_ancestor="true"
-  if [[ -z "$parent_is_ancestor" ]]; then
-    error "The previous commit where the subrepo was updated, '$subrepo_parent' is not an ancestor of HEAD. \n \
-      Did you rebase that commit?\n \
-      Try manually setting subrepo.parent in .gitrepo to the hash of the correct commit."
-  fi
-
   if [[ -n "$subrepo_parent" ]]; then
+
+    # Check if the subrepo parent is an ancestor of HEAD.
+    # For example rebasing the commit that touched the subrepo
+    # could make the commit no longer match.
+    local parent_is_ancestor=
+    git merge-base --is-ancestor "$subrepo_parent" HEAD && parent_is_ancestor="true"
+    if [[ -z "$parent_is_ancestor" ]]; then
+      error "The previous commit where the subrepo was updated, '$subrepo_parent' is not an ancestor of HEAD. \n \
+        Did you rebase that commit?\n \
+        Try manually setting subrepo.parent in .gitrepo to the hash of the correct commit."
+    fi
+
     local prev_commit=
     local ancestor=
     o "Create new commits with parents into the subrepo fetch"