In large repositories with lot of files, the fncache grows more than 100 MB and
reading that whole thing into memory slows things down. Let's not read the whole
thing into memory.
This patch changes fncache loading code to read 1 MB at once. Loading 1 MB at
once saves ~1 sec on perffncacheload for our internal repository. I tried
various values such as 0.5 MB, 5 MB, 10 MB but best results were produced using
1 MB as the chunksize.
On a narrow clone with fncache around 40 MB, this patch saves ~0.04 seconds on
average on perffncacheload.
To test the code, I have coded an extension in test-fncache.t which set
chunksize to 1 byte, and the test passes with that.