destutil: ensure we offer 'hg update' hint when not at head in all cases

In the merge case, we abort if the working copy is not at head, offering to
run 'hg update' in the hint instead. In the rebase case, we do not abort in that
case. Yet if no rebase destination are found, it still make sense to hint the
user about running 'hg update'. So we re-introduce a conditional using this
branch in the 'onheadcheck == False' case.

This will get used on rebase use this function.

mercurial/destutil.py

@@ -235,7 +235,8 b" def _destmergebranch(repo, action='merge"
             branch = ctx.branch()
 
     bheads = repo.branchheads(branch)
-    if onheadcheck and not repo.revs('%ld and %ln', sourceset, bheads):
+    onhead = repo.revs('%ld and %ln', sourceset, bheads)
+    if onheadcheck and not onhead:
         # Case A: working copy if not on a head. (merge only)
         #
         # This is probably a user mistake We bailout pointing at 'hg update'
@@ -267,6 +268,10 b" def _destmergebranch(repo, action='merge"
         elif len(repo.heads()) > 1:
             msg, hint = msgdestmerge['nootherbranchheads'][action]
             msg %= branch
+        elif not onhead:
+            # if 'onheadcheck == False' (rebase case),
+            # this was not caught in Case A.
+            msg, hint = msgdestmerge['nootherheadsbehind'][action]
         else:
             msg, hint = msgdestmerge['nootherheads'][action]
         raise error.NoMergeDestAbort(msg, hint=hint)