fsmonitor: delay updating state until after split index is merged

fsmonitor: delay updating state until after split index is merged

If the fsmonitor extension is used in conjunction with the split index
extension, the set of entries in the index when it is first loaded is
only a subset of the real index.  This leads to only the non-"base"
index being marked as CE_FSMONITOR_VALID.

Delay the expansion of the ewah bitmap until after tweak_split_index
has been called to merge in the base index as well.

The new fsmonitor_dirty is kept from being leaked by dint of being
cleaned up in post_read_index_from, which is guaranteed to be called
after do_read_index in read_index_from.

Signed-off-by: Alex Vandiver <alexmv@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>