git-svn: check HTTPS connection with Python Requests

git-svn will put up the "Reject/Accept" prompt if it encounters a bad HTTPS
certificate.  I could find no way to stop it from doing that.  So instead,
this checks the HTTPS connection with an HTTP HEAD request first.

fdroidserver/common.py

@@ -1007,6 +1007,11 @@ class vcs_gitsvn(vcs):
             if not remote.startswith('https://'):
                 raise VCSException(_('HTTPS must be used with Subversion URLs!'))
 
+            # git-svn sucks at certificate validation, this throws useful errors:
+            import requests
+            r = requests.head(remote)
+            r.raise_for_status()
+
             gitsvn_args.extend(['--', remote, self.local])
             p = self.git(gitsvn_args)
             if p.returncode != 0: