update-tags-encoding: escape '/' and ',' in tags

This is more conflict resistant than the old encoding where tags was a
file with a line per tag.

This commit only handles the "fs/db => Issue" direction.

tools/update-tags-encoding

@@ -1,7 +1,12 @@
 #!/bin/bash
 #
-# Check out the `entomologist-data` branch in a temporary worktree.
-# For each issue with a `tags` file, replace the old-style tags file with a new-style tags dir.
+# Check out the specified branch (or `entomologist-data` if not specified)
+# in a temporary worktree.
+#
+# For each issue with a `tags` file:
+#
+#     replace the old-style tags file with a new-style tags dir
+#
 #     git commit
 
 set -e