Updated Subversion add command again. BEST PRACTICE.

This ...

    svn add --force *

... has the unwanted effect of adding .pycs and other files that should be
ignored.

In other words, no such ...

    svn add --force --ignore-things-that-should-be-ignored *

... exists. Which is a gap in the Subversion client UI that is difficult to
understand.

On the other hand, the collective wisdom of Stack Overflow shows that the
machine-readable output of ...

    svn st

    ? /path/to/first/unversioned/file
    ? /path/to/second/unversioned/file

... should be parsed by hand (with sed, awk, Python, whatever) to create these
commands ...

    svn add /path/to/first/unversioned/file
    svn add /path/to/second/unversioned/file

... which add just those unversioned files that should not be ignored.

This commit implements this logic in the Manager._repository_add_command
property. The output of that property looks like ...

    svn add /path/to/first/unversioned/file && svn add /path/to/second/unversioned/file

... with the double ampersand join of the files-to-be-added.