fetch: use an abort hint where appropriate

hgext/fetch.py

@@ -56,8 +56,8 b" def fetch(ui, repo, source='default', **"
     except error.RepoLookupError:
         branchnode = None
     if parent != branchnode:
-        raise util.Abort(_('working directory not at branch tip '
-                           '(use "hg update" to check out branch tip)'))
+        raise util.Abort(_('working directory not at branch tip'),
+                         hint=_('use "hg update" to check out branch tip'))
 
     wlock = lock = None
     try:

tests/test-fetch.t

@@ -339,7 +339,8 b' pull in change on different branch than '
   marked working directory as branch topic
   (branches are permanent and global, did you want a bookmark?)
   $ hg -R n2 fetch -m merge n1
-  abort: working directory not at branch tip (use "hg update" to check out branch tip)
+  abort: working directory not at branch tip
+  (use "hg update" to check out branch tip)
   [255]
 
 parent should be 0 (fetch did not update or merge anything)