Add support for svn:externals "1.5" style

[eblot]

For now, Trac only supports the old-style syntax.

Subversion 1.5 URLs have the following special prefixes:

../

Relative to the URL of the directory on which the svn:externals property is set

^/

Relative to the root of the repository in which the svn:externals property is versioned

//

Relative to the scheme of the URL of the directory on which the svn:externals property is set

/

Relative to the root URL of the server on which the svn:externals property is versioned

Also, there's a new convention for svn:externals, instead of:

RELPATH [-r<rev>] URL

the mapping can now be written:

[-r<rev>] URL RELPATH

(taken from http://svnbook.red-bean.com/en/1.5/svn.advanced.externals.html)

[eblot]

Note: It's not only about the prefixes, it's also about the parameter order in the external info line. SVN book quoting is incomplete ;-)

[cboos]

Oh brilliant, I missed that part... If I'm not mistaken, looks like we have to check for one of "-.^/" as the starting character, in order to detect the new mode.