Enable the filesystem cache (fscache) in refresh_index().

[benpeart]

On file systems that support it, this can dramatically speed up operations
like add, commit, describe, rebase, reset, rm that would otherwise have to
lstat() every file to "re-match" the stat information in the index to that
of the file system.

On a synthetic repo with 1M files, "git reset" dropped from 52.02 seconds to
14.42 seconds for a savings of 72%.

Signed-off-by: Ben Peart Ben.Peart@microsoft.com

[dscho]

I like where this is going, but this kind of change already caused one serious regression that had to be reverted in one big hurry. So I would like to spend some quality time reviewing this in-depth, and I will most likely find time to do that toward the end of next week.