update: fix crash on bare update when directaccess is enabled

'rev' may be None. An empty 'rev' string shouldn't also be passed to
unhidehashlikerevs().

mercurial/commands.py

@@ -5530,7 +5530,8 b' def update(ui, repo, node=None, rev=None'
 
         # if we defined a bookmark, we have to remember the original name
         brev = rev
-        repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
+        if rev:
+            repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
         ctx = scmutil.revsingle(repo, rev, rev)
         rev = ctx.rev()
         if ctx.hidden():

tests/test-directaccess.t

@@ -162,6 +162,9 b' This should not throw error'
   $ hg up 3
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
+  $ hg up
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
 `hg revert`
 
   $ hg revert -r 28ad74 --all