--- a/mercurial/merge.py	Sat Aug 17 18:28:55 2019 +0900
+++ b/mercurial/merge.py	Wed Aug 21 13:14:39 2019 -0700
@@ -2025,7 +2025,8 @@
                 raise error.Abort(_("outstanding uncommitted merge"))
             ms = mergestate.read(repo)
             if list(ms.unresolved()):
-                raise error.Abort(_("outstanding merge conflicts"))
+                raise error.Abort(_("outstanding merge conflicts"),
+                                  hint=_("use 'hg resolve' to resolve"))
         if branchmerge:
             if pas == [p2]:
                 raise error.Abort(_("merging with a working directory ancestor"

--- a/tests/test-rebase-inmemory.t	Sat Aug 17 18:28:55 2019 +0900
+++ b/tests/test-rebase-inmemory.t	Wed Aug 21 13:14:39 2019 -0700
@@ -506,6 +506,7 @@
   $ hg rebase -s 2 -d 7
   rebasing 2:177f92b77385 "c"
   abort: outstanding merge conflicts
+  (use 'hg resolve' to resolve)
   [255]
   $ hg resolve -l
   U e

--- a/tests/test-resolve.t	Sat Aug 17 18:28:55 2019 +0900
+++ b/tests/test-resolve.t	Wed Aug 21 13:14:39 2019 -0700
@@ -210,12 +210,15 @@
   [1]
   $ hg up 0
   abort: outstanding merge conflicts
+  (use 'hg resolve' to resolve)
   [255]
   $ hg merge 2
   abort: outstanding merge conflicts
+  (use 'hg resolve' to resolve)
   [255]
   $ hg merge --force 2
   abort: outstanding merge conflicts
+  (use 'hg resolve' to resolve)
   [255]
 
 set up conflict-free merge