ENH: pre-commit: Prevent add .txx files.

A transition occurred in the toolkit to move all .txx files to .hxx files.
This prevents accidental additions of or renames to .txx files.

Change-Id: Id4ace9cfca2c56506d93396366d146173e015003

pre-commit

@@ -299,6 +299,22 @@ done
 )
 test -z "$bad" || die "$bad"
 
+#-----------------------------------------------------------------------------
+# Do not allow adding of files with .txx extension.
+diffs_added=$(git diff-index --diff-filter=A --cached $against -- |
+        sed -n '/^:[^:]/ {s/^://;p;}')
+diffs_normal_added=$(echo "$diffs" | grep -v '^...... 160000')
+bad=$(
+test -n "$diffs_normal" && echo "$diffs_normal" |
+while read src_mode dst_mode src_obj dst_obj status file; do
+	if echo "$file" | grep -q -E '\.txx$'; then
+		echo "Attempted to add file $file."
+		echo "Files with the .txx extension are not allowed -- use .hxx instead."
+	fi
+done
+)
+test -z "$bad" || die "$bad"
+
 #-----------------------------------------------------------------------------
 # Merge checks.
 if test -n "$merge_head"; then