Add test for missing parent error

test/rebase.t

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+
+set -e
+
+source test/setup
+
+use Test::More
+
+# Setup foo with 2 branches, one before the subrepo
+# is added and one after so that we can rebase
+# thus destroying the parent in two ways. The first
+# destroys the second parent, but leave a reference to
+# a merge point. The second such that no history of
+# a merge point exists.
+
+clone-foo-and-bar
+
+(
+    cd "$OWNER/foo"
+    git switch -c branch1
+    add-new-files foo1
+    subrepo-clone-bar-into-foo
+    git branch branch2
+    add-new-files foo2
+) &> /dev/null || die
+
+(
+    cd "$OWNER/bar"
+    add-new-files bar2
+    git push
+) &> /dev/null || die
+
+(
+    cd "$OWNER/foo"
+    # Rebasing onto this merge point will still
+    # be able to find the merge point at branch1
+    git subrepo pull bar
+)
+
+(
+    cd "$OWNER/foo"
+    git switch branch2
+    add-new-files foo-branch2
+    git switch branch1
+    git rebase branch2
+) &> /dev/null || die
+
+# Force subrepo to search of the parent SHA,
+# validate it found the prevous merge point
+{
+    output=$(
+      cd "$OWNER/foo"
+      git subrepo clean --force --all
+      catch git subrepo branch bar
+    )
+
+    like "$output" "caused by a rebase" \
+      "subrepo detected merge point"
+}
+
+done_testing 1
+
+teardown