DirCache: Buffer TREE extension to $GIT_DIR

DirCache: Buffer TREE extension to $GIT_DIR

Increase the in-memory buffer for the TREE extension to 5 MiB, and
overflow to $GIT_DIR instead of /tmp.  Using a larger buffer reduces
the chances a repository will overflow and need to spool the extension
to disk.  Using $GIT_DIR allows the TREE extension contents to have
the same file system protections as the final $GIT_DIR/index.

Wrap the entire thing in a try/finally to ensure the temp file is
deleted from disk after the block has finished using it. To avoid
dangling NFS files, LocalFile.destroy() does close the local file
before deleting it.

Change-Id: I8f871181a4689e3ebf0cdd4fd1769333cf7546c3