git-svn: memoize cmt_metadata

Hi,

 I have about 150 "branches = ..." and "tags = ..." lines in my
 .git/config file, and I see a 30x improvement in 'git svn rebase -l'
 when I memoize the cmt_metadata function. (The run time goes
 down from ~150 seconds to ~5 seconds on my machine).

 I see that it was memoized in version:
 * c1927a8 2006-06-27 | git-svn: several graft-branches improvements

 and then removed in version:
 * 396988e 2007-01-13 | git-svn: get rid of Memoize for now...

 Is it safe to re-memoize this function?

 Thanks,
 Lalit Chhabra