Bash completion updates

[Julian Foad]

'./tools/client-side/bash_completion_test' now checks our Bash 
completion on all svn-like commands -- svnadmin, svndumpfilter, svnlook, 
svnrdump, svnsync -- as well as 'svn' itself, as of r1817491. (Not 
svnserve or svnversion, as those have different forms of commands and 
help output.)

I updated the completions for 'svn' and 'svnadmin' commands.
Anyone fancy updating the completions for 'svnlook' and 'svnsync' which 
are still out of date, and 'svnrdump' which is missing?

- Julian