remotephase: fast path newheads computation in simple case (issue5964)

Changeset 88efb7d6bcb6 fixed the logic of `phases.newheads` but greatly
regressed its performance (up to many order of magnitude). The first step to
fix the regression is to exit early when there is no work to do. If there are
no heads to filter or not roots to filter them, we don't have to do any work.

This fixes the regression when talking to an all public changeset. The
performance is even better than before.

pypy, compared to an all public repo
------------------------------------

8eeed92475d5: 0.005758 seconds
88efb7d6bcb6: 0.602517 seconds (x104)
this code:    0.001508 seconds (-74% from base)

mercurial compared to an all public repo
----------------------------------------

8eeed92475d5: 0.000577 seconds
88efb7d6bcb6: 0.185316 seconds (x321)
this code:    0.000150 seconds (-74% from base)

The performance of newheads, when actual computations are required, is fixed
in the next changeset.

mercurial/phases.py

@@ -664,6 +664,8 b' def newheads(repo, heads, roots):'
 
     * `heads`: define the first subset
     * `roots`: define the second we subtract from the first"""
+    if not heads or not roots:
+        return heads
     repo = repo.unfiltered()
     revs = repo.revs('heads(::%ln - (%ln::%ln))', heads, roots, heads)
     return pycompat.maplist(repo.changelog.node, revs)