git-svn: fix mismatched src/dst errors for branch/tag

git-svn: fix mismatched src/dst errors for branch/tag

This fixes the following issue:

$ git svn branch -t --username=svnuser \
  --commit-url=https://myproj.domain.com/svn mytag
Copying http://myproj.domain.com/svn/trunk at r26 to
https://myproj.domain.com/svn/tags/mytag...

Trying to use an unsupported feature: Source and dest appear not to be
in the same repository (src: 'http://myproj.domain.com/svn/trunk';
dst: 'https://myproj.domain.com/svn/tags/mytag')

[ew: shortened subject]

Signed-off-by: Igor Mironov <igor.a.mironov@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>