canonpath: allow canonicalization of non-existant paths

Such as a copy destination; see the added test.

mercurial/scmutil.py

@@ -313,8 +313,8 b' def canonpath(root, cwd, myname, auditor'
             try:
                 name_st = os.stat(name)
             except OSError:
-                break
-            if util.samestat(name_st, root_st):
+                name_st = None
+            if name_st and util.samestat(name_st, root_st):
                 if not rel:
                     # name was actually the same as root (maybe a symlink)
                     return ''

tests/test-symlinks.t

@@ -168,11 +168,12 b' now addremove should remove old files'
   ? foo
   $ hg status ../link
   ? foo
+  $ hg add foo
+  $ hg cp foo "$TESTTMP/link/bar"
+  foo has not been committed yet, so no copy data will be stored for bar.
   $ cd ..
 
 
-
-
   $ hg init b
   $ cd b
   $ ln -s nothing dangling