merge: hint about using `hg resolve` for resolving conflicts

This was suggested by one of our users at Google. Makes sense to me.

Differential Revision: https://phab.mercurial-scm.org/D6755

mercurial/merge.py

@@ -2025,7 +2025,8 b' def update(repo, node, branchmerge, forc'
                 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"

tests/test-rebase-inmemory.t

@@ -506,6 +506,7 b" Retrying without in-memory merge won't l"
   $ 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

tests/test-resolve.t

@@ -210,12 +210,15 b' can not update or merge when there are u'
   [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