Merge branch 'kw/write-index-reduce-alloc'

We used to spend more than necessary cycles allocating and freeing
piece of memory while writing each index entry out.  This has been
optimized.

* kw/write-index-reduce-alloc:
read-cache: avoid allocating every ondisk entry when writing
read-cache: fix memory leak in do_write_index
perf: add test for writing the index

diff --cc Makefile
Simple merge

diff --cc read-cache.c
Simple merge