git-svn: handle multi-init without --trunk, UseSvmProps fixes

git-svn: handle multi-init without --trunk, UseSvmProps fixes

multi-init did not write a svn-remote.<remote>.url config
entry without a --trunk argument.

Also, The svm:mirror property is used by SVN::Mirror to track
the path of the repository that we are mirroring.  We need to
append that to the source (which is (presumably) just the URL of
the repository root).

Lastly, we now look harder for svm:(source|mirror|uuid) properties
in sub and parent directories.  Since our relative path could
be tweaked.

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