synthrepo: create filectx instance in 'filectxfn' callback

I would like to pass the memctx to the memfilectx constructor, but
it's not available where we currently create the memfilectx. It is
available in the 'filectxfn' callback, so let's create the memfilectx
there instead. A later patch will start actually passing the memctx.

Differential Revision: https://phab.mercurial-scm.org/D1669

contrib/synthrepo.py

@@ -369,14 +369,14 b' def synthesize(ui, repo, descpath, **opt'
             while not validpath(path):
                 path = pickpath()
             data = '%s contents\n' % path
-            files[path] = context.memfilectx(repo, path, data)
+            files[path] = data
             dir = os.path.dirname(path)
             while dir and dir not in dirs:
                 dirs.add(dir)
                 dir = os.path.dirname(dir)
 
         def filectxfn(repo, memctx, path):
-            return files[path]
+            return context.memfilectx(repo, path, files[path])
 
         ui.progress(_synthesizing, None)
         message = 'synthesized wide repo with %d files' % (len(files),)
@@ -444,14 +444,12 b' def synthesize(ui, repo, descpath, **opt'
                 for __ in xrange(add):
                     lines.insert(random.randint(0, len(lines)), makeline())
                 path = fctx.path()
-                changes[path] = context.memfilectx(repo, path,
-                                                   '\n'.join(lines) + '\n')
+                changes[path] = '\n'.join(lines) + '\n'
             for __ in xrange(pick(filesremoved)):
                 path = random.choice(mfk)
                 for __ in xrange(10):
                     path = random.choice(mfk)
                     if path not in changes:
-                        changes[path] = None
                         break
         if filesadded:
             dirs = list(pctx.dirs())
@@ -466,9 +464,11 b' def synthesize(ui, repo, descpath, **opt'
                 pathstr = '/'.join(filter(None, path))
             data = '\n'.join(makeline()
                              for __ in xrange(pick(linesinfilesadded))) + '\n'
-            changes[pathstr] = context.memfilectx(repo, pathstr, data)
+            changes[pathstr] = data
         def filectxfn(repo, memctx, path):
-            return changes[path]
+            if path not in changes:
+                return None
+            return context.memfilectx(repo, path, changes[path])
         if not changes:
             continue
         if revs: