worktree: use xsize_t to access file size

worktree: use xsize_t to access file size

To read the "gitdir" file into memory, we stat the file and
allocate a buffer. But we store the size in an "int", which
may be truncated. We should use a size_t and xsize_t(),
which will detect truncation.

An overflow is unlikely for a "gitdir" file, but it's a good
practice to model.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>