git-svn: persistent memoization

git-svn: persistent memoization

Make memoization of the svn:mergeinfo processing functions persistent with
Memoize::Storable so that the memoization tables don't need to be regenerated
every time the user runs git-svn fetch.

The Memoize::Storable hashes are stored in ENV{GIT_DIR}/svn/.caches.

[ew: changed caches path to avoid conflicts with old repos]
[ew: File::Path::{make_path => mkpath} for compatibility]
[ew: line-wrapped at 80 chars]

Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Andrew Myrick <amyrick@apple.com>