Read core.worktree regardless of whether .git was discovered or not

The C Git documentation stated that the core.worktree config was
not read when the .git directory was found implicitly (from the
working directory).

This was not true, and had not been so for a long time. The
documentation has been updated to document the existing behaviour.

Change-Id: If1e81b6a981b9d70e849f24872f01c110e9bc950
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>