read-cache: fix reading of split index

read-cache: fix reading of split index

The split index extension uses ewah bitmaps to mark index entries as
deleted, instead of removing them from the index directly.  This can
result in an on-disk index, in which entries of stage #0 and higher
stages appear, which are removed later when the index bases are merged.

15999d0 read_index_from(): catch out of order entries when reading an
index file introduces a check which checks if the entries are in order
after each index entry is read in do_read_index.  This check may however
fail when a split index is read.

Fix this by moving checking the index after we know there is no split
index or after the split index bases are successfully merged instead.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>