Handle binary files correctly in tools/check-typo

I'm not familiar enough with Subversion to know how properties work
there, but the translation for detecting binary files to git using
attributes is incorrect. Checking for the binary attribute with
check-attr only detects if the file has been *specified* as binary.

Script altered to use a "well-known" plumbing command to detect whether
git regards a particular file as binary. This takes into account
.gitattributes, but also works if the file has been automatically
detected as binary by git.

.gitattributes

@@ -5,9 +5,9 @@
 boot/ocamlc binary
 boot/ocamllex binary
 boot/ocamldep binary
-*.gif
-*.png
-*.tfm
+*.gif binary
+*.png binary
+*.tfm binary
 
 .gitattributes           ocaml-typo=missing-header
 .gitignore               ocaml-typo=missing-header

tools/check-typo

@@ -33,7 +33,7 @@
 # The rule names are the ones shown above in parentheses.
 
 # Built-in exceptions:
-# - Any binary file (i.e. with git attribute "binary")
+# - Any file git identifies as binary
 #   is automatically exempt from all the rules.
 # - Any file whose name matches one of the following patterns is
 #   automatically exempt from all rules
@@ -127,9 +127,20 @@ IGNORE_DIRS="
     if $is_svn || $is_cmd_line; then :; else continue; fi
     svnrules=''
     if $is_svn; then
-      case `git check-attr binary "$f"` in
-        *'binary: set') continue;;
-      esac
+      # Below is a git plumbing command to detect whether git regards a
+      # particular file as binary. This takes into account .gitattributes, but
+      # also works if the file has been automatically detected as binary by git.
+      # EMPTY is the hash of the empty tree (which is specially known to git -
+      # it is automatically included in every repository) as a way to get
+      # `diff-tree` to print the whole tree state; its `--numstat` output then
+      # prints a summary where two dashes in the first two columns indicates a
+      # binary file.
+      #   (See https://git-scm.com/docs/git-diff-tree#_other_diff_formats and
+      #    the documentation for the --numstat option. Commands designated as
+      #    "plumbing" commands in git have stable output intended for parsing)
+      EMPTY=`git hash-object -t tree /dev/null`
+      git diff-tree --numstat $EMPTY HEAD -- "$f" | grep -q "^-[[:blank:]]-" \
+        && continue
       svnrules=`git check-attr ocaml-typo "$f" | sed -e 's/.*: //'`
       case $svnrules in unspecified) svnrules= ;; esac
     fi