hgweb: extract code for emitting multiple changelist records

hgweb: extract code for emitting multiple changelist records

changelistentry() exists so extensions can easily monkeypatch the
function to add additional metadata.

In at least one case at Mozilla, we have an extension doing this
where performance would greatly benefit if we were able to "batch
fetch" metadata for all revisions that will eventually have their
data set.

By extracting the logic for "obtain records for N revisions" into
a standalone function, we make it possible for a monkeypatched
function to more efficiently bulk fetch data for N revisions.

Differential Revision: https://phab.mercurial-scm.org/D3573

--- a/mercurial/hgweb/webcommands.py	Wed May 16 22:44:27 2018 -0400
+++ b/mercurial/hgweb/webcommands.py	Thu May 17 12:23:38 2018 -0700
@@ -398,14 +398,8 @@
         revs = []
         if pos != -1:
             revs = web.repo.changelog.revs(pos, 0)
-        curcount = 0
-        for rev in revs:
-            curcount += 1
-            if curcount > revcount + 1:
-                break
 
-            entry = webutil.changelistentry(web, web.repo[rev])
-            entry['parity'] = next(parity)
+        for entry in webutil.changelistentries(web, revs, revcount, parity):
             yield entry
 
     if shortlog:

--- a/mercurial/hgweb/webutil.py	Wed May 16 22:44:27 2018 -0400
+++ b/mercurial/hgweb/webutil.py	Thu May 17 12:23:38 2018 -0700
@@ -457,6 +457,22 @@
     )
     return entry
 
+def changelistentries(web, revs, maxcount, parityfn):
+    """Emit up to N records for an iterable of revisions."""
+    repo = web.repo
+
+    count = 0
+    for rev in revs:
+        if count >= maxcount:
+            break
+
+        count += 1
+
+        entry = changelistentry(web, repo[rev])
+        entry['parity'] = next(parityfn)
+
+        yield entry
+
 def symrevorshortnode(req, ctx):
     if 'node' in req.qsparams:
         return templatefilters.revescape(req.qsparams['node'])