Show clearer error message when subrepo_parent is not an ancestor of HEAD

Before, there would be a cryptic error about invalid parameters,
because prev_commit would never get set.

This situation can occur when the commit that last touched the subrepo
has been rebased.

Author: OskiKervinen-MF

lib/git-subrepo

@@ -744,7 +744,20 @@ subrepo:branch() {
   local first_gitrepo_commit=
 
   o "Subrepo parent: $subrepo_parent"
+
   if [[ $subrepo_parent ]]; then
+
+    # Check if the subrepo parent is an ancestor of HEAD.
+    # For example rebasing the commit that touched the subrepo
+    # could make the commit no longer match.
+    local parent_is_ancestor=
+    git merge-base --is-ancestor "$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"