git-svn: multiple fetch/branches/tags keys are supported

git-svn: multiple fetch/branches/tags keys are supported

"git svn" can be configured to use multiple fetch, branches, and tags
refspecs by passing multiple --branches or --tags options at init time
or editing the configuration file later, which can be handy when
working with messy Subversion repositories.  Add a note to the
configuration section documenting how this works.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Eric Wong <redacted>