git-svn: cache only mergeinfo revisions
git-svn: cache only mergeinfo revisions

This should reduce excessive memory usage from the new mergeinfo
caches without hurting performance too much, assuming reasonable
latency to the SVN server.

Cc: Hin-Tak Leung <ec51f4bc3298ae58770617c0319da1091a2df7dd@users.sourceforge.net>
Suggested-by: Jakob Stoklund Olesen <9e01571d0adae2c18b4ca0578f54335c0bc01b66@2pi.dk>
Signed-off-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git