log: bypass file scan part of fastpath when no files

This avoids loading dirstate parents, looking up p1 rev, and loading
p1 manifest to match against an empty matcher.

mercurial/cmdutil.py

@@ -1006,7 +1006,7 b' def walkchangerevs(repo, match, opts, pr'
         wanted = set(revs)
     copies = []
 
-    if not slowpath:
+    if not slowpath and match.files():
         # We only have to read through the filelog to find wanted revisions
 
         minrev, maxrev = min(revs), max(revs)