fix hg help <ext> for extension that do not define any command
solve
issue462
mq: fix strip failing to unlink a bundle
os.unlink doesn't know how to deal with urls
symlinks: minimal support for symlinks in merge/update
This will get angry, confused, and sad if you try to merge a symlink
with a regular file.
symlinks: add flags param to wwrite
Now wwrite can set symlink and exec for files