Fix binary headers in formatted patches (#56)

Include file names in the header (now that we can actually parse them)
and fix a bad find-and-replace that changed "differ" to "fmer". Add a
new test to verify that binary files without data format correctly.

gitdiff/testdata/string/binary_modify_nodata.patch

@@ -0,0 +1,3 @@
+diff --git a/file.bin b/file.bin
+index a7f4d5d..bdc9a70 100644
+Binary files a/file.bin and b/file.bin differ