git-svn: loosen config globs limitations

Expand the area of globs applicability for branches and tags
in git-svn. It is now possible to use globs like 'a*e', or 'release_*'.
This allows users to avoid long lines in config like:

branches = branches/{release_20,release_21,release_22,...}

In favor of:

branches = branches/release_*

[ew: amended commit message, minor formatting and style fixes]

Signed-off-by: Victor Leschuk <vleschuk@accesssoftek.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>