git-send-email: bug with sendemail.multiedit

Bonjour Pierre! ... and all git developers!

I think there is a bug with git-send-email.perl's evaluation of the sendemail.multiedit config variable.

I was only able to make the "do_edit()" function detect it as false by setting the variable to "0" instead
of "false", like so:

git config --global sendemail.multiedit 0

otherwise do_edit evaluates it as true and invokes the editor with all files as argument.

All other git config boolean variables are set to either "true" or "false", not "0" or "1".

Not being too familiar with the perl language, I don't know how to fix this without spending an hour, which
is probably the amount of time I already spent narrowing the problem down already. So I leave this into
more capable hands.

cheers.