Speed-up recursive by flushing index only once for all entries
The merge-recursive implementation in C inherited the invariant that the on-file index file is written out and later read back after any index operations and writing trees from the original Python implementation. But it was only because the original implementation worked at the scripting level.
There is no need to write out the index file after handling every path.
Signed-off-by: Alex Riesen <email@example.com> Signed-off-by: Junio C Hamano <firstname.lastname@example.org>