localrepo: validate directories before creating any

There is no meaningful change in behavior because wdir would
already exist in the case where we raised RepoError. But I think
the code is easier to read if we do all validation first then
take actions with side-effects.

Differential Revision: https://phab.mercurial-scm.org/D4706

mercurial/localrepo.py

@@ -2798,13 +2798,14 b' def createrepository(ui, path, createopt'
     requirements = newreporequirements(ui, createopts=createopts)
 
     wdirvfs = vfsmod.vfs(path, expandpath=True, realpath=True)
-    if not wdirvfs.exists():
-        wdirvfs.makedirs()
 
     hgvfs = vfsmod.vfs(wdirvfs.join(b'.hg'))
     if hgvfs.exists():
         raise error.RepoError(_('repository %s already exists') % path)
 
+    if not wdirvfs.exists():
+        wdirvfs.makedirs()
+
     hgvfs.makedir(notindexed=True)
 
     if b'store' in requirements: