diff: use fctx.isbinary() to test binary

The end goal is to avoid calling fctx.data() when unnecessary. For example,
if diff.nobinary=1 and files are binary, the expected behavior is to print
"Binary file has changed". That could avoid reading fctx.data() sometimes.

This is mainly to enable an external LFS extension to skip expensive binary
file loading sometimes (read: most of the time with diff.nobinary=1 and
diff.text=0), without any behavior changes to mercurial (i.e. whether a file
is LFS or not does not change any behavior, LFS could be 100% transparent to
users).

mercurial/patch.py

@@ -2523,19 +2523,23 b' def trydiff(repo, revs, ctx1, ctx2, modi'
     for f1, f2, copyop in _filepairs(modified, added, removed, copy, opts):
         content1 = None
         content2 = None
+        fctx1 = None
+        fctx2 = None
         flag1 = None
         flag2 = None
         if f1:
-            content1 = getfilectx(f1, ctx1).data()
+            fctx1 = getfilectx(f1, ctx1)
+            content1 = fctx1.data()
             if opts.git or losedatafn:
                 flag1 = ctx1.flags(f1)
         if f2:
-            content2 = getfilectx(f2, ctx2).data()
+            fctx2 = getfilectx(f2, ctx2)
+            content2 = fctx2.data()
             if opts.git or losedatafn:
                 flag2 = ctx2.flags(f2)
         binary = False
         if opts.git or losedatafn:
-            binary = util.binary(content1) or util.binary(content2)
+            binary = any(f.isbinary() for f in [fctx1, fctx2] if f is not None)
 
         if losedatafn and not opts.git:
             if (binary or