Rewrite GitMoveDeleteHook to use DirCacheBuilder

Rewrite GitMoveDeleteHook to use DirCacheBuilder

The new DirCache API can read and write an index file faster than
the older GitIndex API can.  Its slightly more verbose to use but
operations will execute more quickly, especially when we are forced
to perform these single path updates by Eclipse.

When we move a file we maintain its stage status.  If the file has
been modified on disk but is not yet staged into the index (or it
was modified since staging) we leave the index alone, retaining
that staged state.

If we are moving within the same index file (same repository) we
only write the index out once.  This is a common case as most of
the renames performed by users will be within the same project,
and the same Git repository.  It is rare for a rename to wind up
spanning multiple repositories.

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