parsers: remove unreachable and invalid code in index_ancestors

The function normally returns a list. Returning a single element instead of a
list with one element would be weird.
ancestors: remove unnecessary handling of 'left'

If one of the initial nodes also is an ancestor then that most be the only
ancestor. There is no need for additional bookkeeping.
localrepo: commit: avoid calling expensive ancestor function when p1 is nullrev
revlog: introduce commonancestors method for getting all common ancestor heads
push: provide a hint when no paths in configured (issue3692)

When user type "hg push" command then this patch helps user by
providing hint if no default path is configured.

Second patch is the test coverage, to test the change behaviour of
first patch.

--HG--
extra : amend_source : 74c3345e2dc3a57cd97bc09822f676891130b77c
scmutil: changed revrange code not to use append

Removed one call to the append method