trydiff: extract 'date2' variable like existing 'date1'

Note that there is a comment saying "ctx2 date may be dynamic". The
comment was introduced in dccb83241dd0 (patch: use contexts for diff,
2006-12-25), but it seems like it stopped being dynamic in that very
changeset -- before that changeset, the date seems to have been the
file's mtime, but after the changeset, it seems to be the changeset's
timestamp (current time for workingctx). Since no one seems to have
missed the "dynamicness", let's simplify and extract a date2 for
symmetry with date1.

mercurial/patch.py

@@ -1784,6 +1784,7 b' def trydiff(repo, revs, ctx1, ctx2, modi'
         return line
 
     date1 = util.datestr(ctx1.date())
+    date2 = util.datestr(ctx2.date())
     man1 = ctx1.manifest()
 
     gone = set()
@@ -1878,9 +1879,8 b' def trydiff(repo, revs, ctx1, ctx2, modi'
                     addindexmeta(header, [gitindex(to), gitindex(tn)])
             else:
                 text = mdiff.unidiff(to, date1,
-                                    # ctx2 date may be dynamic
-                                    tn, util.datestr(ctx2.date()),
-                                    join(a), join(b), opts=opts)
+                                     tn, date2,
+                                     join(a), join(b), opts=opts)
             if header and (text or len(header) > 1):
                 yield ''.join(header)
             if text: