changelog: lazily parse files

More of the same.

Again, modest revset performance wins:

author(mpm)
0.896565
0.822961
0.805156

desc(bug)
0.887169
0.847054
0.798101

date(2015)
0.878797
0.811613
0.786689

extra(rebase_source)
0.865446
0.797756
0.777408

 author(mpm) or author(greg)
1.801832
1.668172
1.626547

author(mpm) or desc(bug)
1.812438
1.677608
1.613941

date(2015) or branch(default)
0.968276
0.896032
0.869017

mercurial/changelog.py

@@ -153,7 +153,7 b' class changelogrevision(object):'
     __slots__ = (
         '_rawdateextra',
         '_rawdesc',
-        'files',
+        '_rawfiles',
         '_rawmanifest',
         '_rawuser',
     )
@@ -196,8 +196,12 b' class changelogrevision(object):'
         nl3 = text.index('\n', nl2 + 1)
         self._rawdateextra = text[nl2 + 1:nl3]
 
-        l = text[:doublenl].split('\n')
-        self.files = l[3:]
+        # The list of files may be empty. Which means nl3 is the first of the
+        # double newline that precedes the description.
+        if nl3 == doublenl:
+            self._rawfiles = None
+        else:
+            self._rawfiles = text[nl3 + 1:doublenl]
 
         return self
 
@@ -242,6 +246,13 b' class changelogrevision(object):'
         return decodeextra(raw)
 
     @property
+    def files(self):
+        if self._rawfiles is None:
+            return []
+
+        return self._rawfiles.split('\n')
+
+    @property
     def description(self):
         return encoding.tolocal(self._rawdesc)