glossary: mention 'treeish' as an alternative to 'tree-ish'

The documentation contains a mix of the two spellings, so include both
in the glossary so that a search for either will lead to the
definition.

Signed-off-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -486,7 +486,7 @@ should not be combined with other pathspec.
 	with refs to the associated blob and/or tree objects. A
 	<<def_tree,tree>> is equivalent to a <<def_directory,directory>>.
 
-[[def_tree-ish]]tree-ish::
+[[def_tree-ish]]tree-ish (also treeish)::
 	A <<def_ref,ref>> pointing to either a <<def_commit_object,commit
 	object>>, a <<def_tree_object,tree object>>, or a <<def_tag_object,tag
 	object>> pointing to a tag or commit or tree object.