git-svn: fix several small bugs, enable branch optimization

git-svn: fix several small bugs, enable branch optimization

Share the repack counter between branches when doing
multi-fetch.

Pass the -d flag to git repack by default.  That's the
main reason we will want automatic pack generation, to
save space and improve disk cache performance.  I won't
add -a by default since it can generate extremely large
packs that make RAM-starved systems unhappy.

We no longer generate the .git/svn/$GIT_SVN_ID/info/uuid
file, either.  It was never read in the first place.

Check for and create .rev_db if we need to during fetch (in case
somebody manually blew away their .rev_db and wanted to start
over.  Mainly makes debugging easier).

Croak with $? instead of $! if there's an error closing pipes

Quiet down some of the chatter, too.

Signed-off-by: Eric Wong <normalperson@yhbt.net>