git-svn: allow subset of branches/tags to be specified in glob spec

git-svn: allow subset of branches/tags to be specified in glob spec

For very large projects it is useful to be able to clone a subset of the
upstream SVN repo's branches. Allow for this by letting the left-side of
the branches and tags glob specs contain a brace-delineated comma-separated
list of names. e.g.:

	branches = branches/{red,green}/src:refs/remotes/branches/*

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>