git-svn: don't allow globs to match regular files

git only tracks the histories of full directories, not
that of individual files.  Sometimes, SVN users will
place[1] a regular file in the directory designated
for subdirectories of branches or tags.

Thanks to jrockway on #git for pointing this out.

[1] mistakenly or otherwise, such as a README

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>