trydiff: set filename variables to None even when not opts.git or losedatafn

There is not much left of the first block "if opts.git or losedatafn"
block now. The next patch will move the call to getfilectx() out of
that block. We will then be using the defined-ness of 'f1' to tell
whether the file existed in ctx1 (and under what name). We will need
this information whether or not opts.git or losedatafn was set, so
just remove that guard. The only operation in the block that is not
cheap is the call to getfilectx(), but that has an extra 'if opts.git'
guard already.

--ignore-space-change proves that only 'if opts.git or losedatafn:'
  was removed.

mercurial/patch.py

@@ -1780,7 +1780,6 b' def trydiff(repo, revs, ctx1, ctx2, modi'
         if f not in removedset:
             content2 = getfilectx(f, ctx2).data()
         f1, f2 = f, f
-        if opts.git or losedatafn:
             if f in addedset:
                 f1 = None
                 if f in copy: