git-svn: disable _rev_list memoization

git-svn: disable _rev_list memoization

This memoization appears unneeded as the check_cherry_pick2 cache is
in front of it does enough.

With this change applied, importing from local svn+ssh and http copies
of the R repo[1] takes only 2:00 (2 hours) on my system and the git-svn
process never uses more than 60MB RSS on my x86-64 GNU/Linux system[2].
This 60M measurement is only for the git-svn Perl process itself and
does not include memory used by git subprocesses accessing large packs
(subprocess memory usage _is_ measured by my time(1) tool).

Before this change, an import took longer (2:20) on svn+ssh:// but
git-svn used around 240MB during the imports.  Worse yet, git-svn
ballooned to over 400M when writing out the cache to the filesystem.

I also tried removing memoization for `has_no_changes', too, but a
local copy of the R repository(*) was not close to finishing within
10 hours on my system.

[1] http://svn.r-project.org/R
[2] file:// repos causes libsvn to use more memory internally

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Cc: Hin-Tak Leung <htl10@users.sourceforge.net>