git-svn: fix regex to remove "tail" from svn tags

git-svn: fix regex to remove "tail" from svn tags

Fix a regular expression used to remove the revision from the
end of an svn tag or branch name.  The regex did not account for
any "tail" (dashes) that may have been added to the end of the
tag name (which first appeared in v1.4.1-rc2~11).  If not fixed,
tags with names like "tags/mytag@5--@2" may be created.

Signed-off-by: David D. Kilzer <ddkilzer@kilzer.net>
Acked-by: Eric Wong <normalperson@yhbt.net>