read-cache: speed up add_index_entry during checkout

read-cache: speed up add_index_entry during checkout

Teach add_index_entry_with_check() to see if the path
of the new item is greater than the last path in the
index array before attempting to search for it.

During checkout, merge_working_tree() populates the new
index in sorted order, so this change will save a binary
lookups per file.  This preserves the original behavior
but simply checks the last element before starting the
search.

This helps performance on very large repositories.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>