speed up hg log --patch

Changing dodiff to read the manifest/changelog for node1 before calling
repo.update allows us to take advantage of the revlog revision cache.

Before this patch and my previous "speed up hg log --debug" patch, when
using hg log -p to display three revisions (A, B and C), dodiff and
repo.changes would end up reading the manifests in this order:

 B A B A C B C B

With both patches, this order becomes:

 A A B B B B C C

(This considers only dodiff and repo.changes.  I'm not sure how other
parts of hg log enter the picture.)

The speed up will depend on the revisions being displayed.  (All
"before" times already have my previous "speed up hg log --debug" patch
applied.)

hg repo (tip = 414e81ae971f).  hg log -p
         before           after
real    0m50.981s       0m45.279s
user    0m47.930s       0m42.560s
sys     0m2.526s        0m2.523s
output size: 6917897 bytes

kernel repo (tip = 9d4e135960ed).  hg log -p -l64
         before           after
real    2m14.995s       1m45.025s
user    2m9.509s        1m33.900s
sys     0m3.663s        0m2.942s
output size: 31497621 bytes

same kernel repo.  hg log -p -l64 -r c84c2069592f:0
         before           after
real    1m48.045s       1m0.076s
user    1m44.094s       0m58.492s
sys     0m2.603s        0m1.103s
output size: 197983 bytes

c84c2069592f was the tip of a 10 day old kernel repo that I had lying
around and was where I first tested this patch.  For some weird
coincidence it's also a place where the patch makes a huge difference.

mercurial/commands.py

@@ -276,6 +276,13 b' def make_file(repo, r, pat, node=None,'
 
 def dodiff(fp, ui, repo, node1, node2, files=None, match=util.always,
            changes=None, text=False, opts={}):
+    if node1:
+        # reading the data for node1 early allows it to play nicely
+        # with repo.changes and the revlog cache.
+        change = repo.changelog.read(node1)
+        mmap = repo.manifest.read(change[0])
+        date1 = util.datestr(change[2])
+
     if not changes:
         changes = repo.changes(node1, node2, files, match=match)
     modified, added, removed, deleted, unknown = changes
@@ -296,6 +303,9 b' def dodiff(fp, ui, repo, node1, node2, f'
         date2 = util.datestr()
         if not node1:
             node1 = repo.dirstate.parents()[0]
+            change = repo.changelog.read(node1)
+            mmap = repo.manifest.read(change[0])
+            date1 = util.datestr(change[2])
         def read(f):
             return repo.wread(f)
 
@@ -305,10 +315,6 b' def dodiff(fp, ui, repo, node1, node2, f'
         hexfunc = ui.verbose and hex or short
         r = [hexfunc(node) for node in [node1, node2] if node]
 
-    change = repo.changelog.read(node1)
-    mmap = repo.manifest.read(change[0])
-    date1 = util.datestr(change[2])
-
     diffopts = ui.diffopts()
     showfunc = opts.get('show_function') or diffopts['showfunc']
     ignorews = opts.get('ignore_all_space') or diffopts['ignorews']