copies: avoid calling matcher if matcher.always()

When storing copy information in the changesets
(experimental.copies.read-from=changeset-only), this patch speeds up

  hg debugpathcopies FENNEC_58_0_2_BUILD1 FIREFOX_59_0b8_BUILD2

from 5.9s to 4.7s. At the start of this series (b162229e), that
command took 18min.

Differential Revision: https://phab.mercurial-scm.org/D6422

mercurial/copies.py

@@ -270,6 +270,7 b' def _changesetforwardcopies(a, b, match)'
     # came from.
     work = [(r, 1, {}) for r in roots]
     heapq.heapify(work)
+    alwaysmatch = match.always()
     while work:
         r, i1, copies1 = heapq.heappop(work)
         if work and work[0][0] == r:
@@ -280,7 +281,7 b' def _changesetforwardcopies(a, b, match)'
             # TODO: perhaps this filtering should be done as long as ctx
             # is merge, whether or not we're tracing from both parent.
             for dst in allcopies:
-                if not match(dst):
+                if not alwaysmatch and not match(dst):
                     continue
                 # Unlike when copies are stored in the filelog, we consider
                 # it a copy even if the destination already existed on the
@@ -306,7 +307,7 b' def _changesetforwardcopies(a, b, match)'
                 assert r == childctx.p2().rev()
                 parent = 2
                 childcopies = childctx.p2copies()
-            if not match.always():
+            if not alwaysmatch:
                 childcopies = {dst: src for dst, src in childcopies.items()
                                if match(dst)}
             # Copy the dict only if later iterations will also need it