git-svn: fix useSvmProps, hopefully for the last time

git-svn: fix useSvmProps, hopefully for the last time

svm:mirror is not useful at all for us.  Parts of the old unit
test were broken and based on my misunderstanding of the
svm:mirror property.

When we read svm:source; make sure we correctly handle the '!'
in it: it is used to separate the path of the repository root
from the virtual path within the repository.  We don't need
to make that distinction, honestly!

We also ensure that subdirectories are also mirrored with the
correct URL if we're using useSvmProps.

We have a new test that uses dumped repo that was really
created using SVN::Mirror to avoid ambiguities and
mis-understandings about the svm: properties.

Note: trailing whitespace in the svm.dump file is unfortunately
a reality and required by SVN; so please ignore it when applying
this patch.

Also, ensure that the -R/--remote/--svn-remote flag is always
in effect if explicitly passed via the command-line.  This
allows us to track logically different mirrors sharing the
same URL (probably common with SVN::Mirror/SVK users).

Signed-off-by: Eric Wong <normalperson@yhbt.net>