identify: avoid a crash when given '-r wdir()'

The crash was 'NoneType is not subscriptable' in hexfunc(ctx.node()), because
the node for wdir() is None.  This can be avoided simply by detecting 'wdir()'
and taking the existing path for no given revision.

mercurial/commands.py

@@ -4064,7 +4064,9 b' def identify(ui, repo, source=None, rev='
             if bm:
                 output.append(bm)
     else:
-        if not rev:
+        ctx = scmutil.revsingle(repo, rev, None)
+
+        if ctx.rev() is None:
             ctx = repo[None]
             parents = ctx.parents()
             changed = ""
@@ -4079,7 +4081,6 b' def identify(ui, repo, source=None, rev='
                 output.append("%s%s" %
                   ('+'.join([str(p.rev()) for p in parents]), changed))
         else:
-            ctx = scmutil.revsingle(repo, rev)
             if default or id:
                 output = [hexfunc(ctx.node())]
             if num:

tests/test-tags.t

@@ -47,6 +47,8 b' Setup:'
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg identify
   acb14030fe0a tip
+  $ hg identify -r 'wdir()'
+  acb14030fe0a tip
   $ cacheexists
   tag cache exists
 No fnodes cache because .hgtags file doesn't exist
@@ -174,6 +176,8 b' Create a branch:'
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg id
   acb14030fe0a+ first
+  $ hg id -r 'wdir()'
+  acb14030fe0a+ first
   $ hg -v id
   acb14030fe0a+ first
   $ hg status