convert: accept Subversion 'file:///c%3A/svnrepo' syntax on Windows

Subversion can handle ':' quoted as '%3A' but urllib.url2pathname can't and
Mercurial thus rejected some valid subversions URLs.

This particular case will now be handled by some preprocessing before handing
it over to urllib.url2pathname.

This is tested by 0413f68da85c when test-convert-svn-source.t and
test-convert-svn-move.t can be run on Windows.

hgext/convert/subversion.py

@@ -189,6 +189,9 b' def issvnurl(ui, url):'
     try:
         proto, path = url.split('://', 1)
         if proto == 'file':
+            if (os.name == 'nt' and path[:1] == '/' and path[1:2].isalpha()
+                and path[2:6].lower() == '%3a/'):
+                path = path[:2] + ':/' + path[6:]
             path = urllib.url2pathname(path)
     except ValueError:
         proto = 'file'