Rename Repository getWorkDir to getWorkTree

This better matches with the name used in the environment
(GIT_WORK_TREE), in the configuration file (core.worktree),
and in our builder object.

Since we are already breaking a good chunk of other code
related to repository access, and this fairly easy to fix
in an application's code base, I'm not going to offer the
wrapper getWorkDir() method.

Change-Id: Ib698ba4bbc213c48114f342378cecfe377e37bb7
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>