This patch adds a remote.*.mirror configuration option that,
when set, automatically puts git-push in --mirror mode for that
remote.

Furthermore, the option is set automatically by `git remote
add --mirror'.

The code in remote.c to parse remote.*.skipdefaultupdate
had a subtle problem: a comment in the code indicated that
special care was needed for boolean options, but this care was
not used in parsing the option.  Since I was touching related
code, I did this fix too.

[jc: and I further fixed up the "ignore boolean" code.]
Signed-off-by: NPaolo Bonzini <bonzini@gnu.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>