Refine how tree[subscript] is introduced

This drops the newly introduced precision in Python terminology,
likening usage to that of a list or dict (experienced Python users
will already know about sequences and mappings, and inexperienced
users will, with these terms, be able to understand what is said).

Note that this does not undo the broader effect of the previous
commit, which is to make clear that subscripting a tree supports
both list-like and dict-like usage, not just list-like usage.

Author: EliahKagan

Diff for: git/objects/tree.py

@@ -164,7 +164,7 @@ class Tree(IndexObject, git_diff.Diffable, util.Traversable, util.Serializable):
     R"""Tree objects represent an ordered list of :class:`~git.objects.blob.Blob`\s and
     other :class:`Tree`\s.
 
-    Subscripting is supported, as with a mapping or sequence:
+    Subscripting is supported, as with a list or dict:
 
     * Access a specific blob using the ``tree["filename"]`` notation.
     * You may likewise access by index, like ``blob = tree[0]``.