update: do not pass in user revspec as default destination (issue6044)

When the revsingle() was introduced at 61c0df2b089a, it couldn't handle
revspec=0 (not '0') properly. That's probably why the default was set to
rev.

This is technically BC since "hg update ''" was identical to "hg update '.'"
whereas "hg update -r ''" is "hg update", but I believe that's a bug given
no test fails with this change.

mercurial/commands.py

@@ -6038,7 +6038,7 b' def update(ui, repo, node=None, **opts):'
         brev = rev
         if rev:
             repo = scmutil.unhidehashlikerevs(repo, [rev], 'nowarn')
-        ctx = scmutil.revsingle(repo, rev, rev)
+        ctx = scmutil.revsingle(repo, rev, default=None)
         rev = ctx.rev()
         hidden = ctx.hidden()
         overrides = {('ui', 'forcemerge'): opts.get(r'tool', '')}

tests/test-simple-update.t

@@ -57,6 +57,26 b' update to rev 0 with a date'
   abort: you can't specify a revision and a date
   [255]
 
+update to default destination (with empty revspec)
+
+  $ hg update -q null
+  $ hg update
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  30aff43faee1 tip
+
+  $ hg update -q null
+  $ hg update -r ''
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  30aff43faee1 tip
+
+  $ hg update -q null
+  $ hg update ''
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg id
+  30aff43faee1 tip
+
   $ cd ..
 
 update with worker processes