[PATCH] push: do not require --force to push bookmarked head (issue2372)

[Prasoon Shukla]

A couple of things. The error was caused by the discovery.checkheads()
which raised the error. We can check inside discovery.checkheads()
whether the new head is bookmarked. But, just checking whether the
pushed head is bookmarked will also allow for the case where someone
can implicitly (and maybe without their knowledge) push a bookmarked
head upstream. So, we need to be know inside discovery.checkheads
whether the user is indeed trying to push the bookmark. That is why we
have added a new flag, bkmark, that would allow discovery.checkheads
to determine exactly this.