Allow tools/check-typo to analyse non-HEAD commits

Three alterations to tools/check-typo:

1. Binary check may be on any commit where the default is HEAD
2. .gitattributes may be read from any commit, rather than just the
   working tree
3. Detection of files under version control may be relative to a
   specific commit, rather than relying on git ls-files

tools/check-typo

@@ -91,7 +91,8 @@ export LC_ALL=C
 # Special case for recursive call from the find command (see IGNORE_DIRS).
 case "$1" in
   --check-prune)
-    case `git check-attr $OCAML_CT_CA_FLAG ocaml-typo "$2" 2>/dev/null` in
+    case `env $OCAML_CT_GIT_INDEX git check-attr $OCAML_CT_CA_FLAG \
+                                                 ocaml-typo "$2" 2>/dev/null` in
       *prune*) echo "INFO: pruned directory $2 (ocaml-typo=prune)" >&2; exit 0;;
       *) exit 3;;
     esac;;
@@ -114,6 +115,13 @@ while : ; do
 done
 
 OCAML_CT_CAT=${OCAML_CT_CAT:-cat}
+OCAML_CT_LS_FILES=${OCAML_CT_LS_FILES:-git ls-files}
+OCAML_CT_HEAD=${OCAML_CT_HEAD:-HEAD}
+if [ -z "${OCAML_CT_GIT_INDEX+x}" ] ; then
+  OCAML_CT_GIT_INDEX=
+else
+  OCAML_CT_GIT_INDEX="GIT_INDEX_FILE=$OCAML_CT_GIT_INDEX"
+fi
 
 IGNORE_DIRS="
   -name .git -prune -o
@@ -126,7 +134,7 @@ IGNORE_DIRS="
   esac
 ) | (
   while read f; do
-    case `git ls-files "$f" 2>&1` in
+    case `$OCAML_CT_LS_FILES "$f" 2>&1` in
       "") is_svn=false;;
       *) is_svn=true;;
     esac
@@ -149,9 +157,10 @@ IGNORE_DIRS="
       #    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 $OCAML_CT_CA_FLAG ocaml-typo "$f" \
+      git diff-tree --numstat $EMPTY $OCAML_CT_HEAD -- "$f" \
+        | grep -q "^-[[:blank:]]-" && continue
+      svnrules=`env $OCAML_CT_GIT_INDEX git check-attr $OCAML_CT_CA_FLAG \
+                                                       ocaml-typo "$f" \
                   | sed -e 's/.*: //'`
       case $svnrules in unspecified) svnrules= ;; esac
     fi