trydiff: read file data in only one place

This moves getfilectx() out of the initial block in the loop, leaving
that block to be only about finding pairs of filenames in ctx1 and
ctx2 to diff.

mercurial/patch.py

@@ -1772,13 +1772,7 b' def trydiff(repo, revs, ctx1, ctx2, modi'
         if f not in ctx1:
             addedset.add(f)
     for f in sorted(modified + added + removed):
-        content1 = None
-        content2 = None
         copyop = None
-        if f not in addedset:
-            content1 = getfilectx(f, ctx1).data()
-        if f not in removedset:
-            content2 = getfilectx(f, ctx2).data()
         f1, f2 = f, f
         if f in addedset:
             f1 = None
@@ -1790,7 +1784,6 b' def trydiff(repo, revs, ctx1, ctx2, modi'
                         gone.add(f1)
                     else:
                         copyop = 'copy'
-                    content1 = getfilectx(f1, ctx1).data()
         elif f in removedset:
             f2 = None
             if opts.git:
@@ -1799,6 +1792,12 b' def trydiff(repo, revs, ctx1, ctx2, modi'
                     and copy[copyto[f]] == f):
                     continue
 
+        content1 = None
+        content2 = None
+        if f1:
+            content1 = getfilectx(f1, ctx1).data()
+        if f2:
+            content2 = getfilectx(f2, ctx2).data()
         flag1 = None
         flag2 = None
         binary = False