Rename GitilesView's TreePath to PathPart

We originally chose "TreePath" instead of "Path" to distinguish between
the whole URL path, e.g. /repo/name/+/tree/path, from the portion of the
path referring to the tree, e.g. /tree/path. However, this portion no
longer necessarily refers to a tree, e.g. in /+refs/heads?format=TEXT.
Change the name appropriately.

Change-Id: Iefa930e60f5780bc9d9446cf3e5b13fa90e9d926