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.
win32: remove uneeded usage of _STARTF_USESHOWWINDOW
spawndetached() was the only user of _STARTF_USESHOWWINDOW and it creates the
process with _CREATE_NO_WINDOW anyway. If the process has no window, then
there is nothing to hide.