git-svn: add join_paths() to safely concatenate paths

git-svn: add join_paths() to safely concatenate paths

Otherwise you might wind up with things like...

    my $path1 = undef;
    my $path2 = 'foo';
    my $path = $path1 . '/' . $path2;

creating '/foo'.  Or this...

    my $path1 = 'foo/';
    my $path2 = 'bar';
    my $path = $path1 . '/' . $path2;

creating 'foo//bar'.

Could have used File::Spec, but I'm shying away from it due to SVN
1.7's pickiness about paths.  Felt it would be better to have our own
we can control completely.

[ew: commit title]

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