log: speed up hg log <file|folder>

This patch makes hg log <file|folder> faster by using changelog.readfiles
instead of changelog.read.
On our large repos for hg log <file|folder> -l5 operations that were taking:
- ~8s I see a 25% improvement
- ~15s, I see a 35% improvement
For recently modified folder/file, the difference is negligible as we don't
have to consider many revisions.

mercurial/revset.py

@@ -1166,13 +1166,13 b' def _matchfiles(repo, subset, x):'
 
     # This directly read the changelog data as creating changectx for all
     # revisions is quite expensive.
-    getchangeset = repo.changelog.read
+    getfiles = repo.changelog.readfiles
     wdirrev = node.wdirrev
     def matches(x):
         if x == wdirrev:
             files = repo[x].files()
         else:
-            files = getchangeset(x)[3]
+            files = getfiles(x)
         for f in files:
             if m(f):
                 return True