Document usepushurl in the man page
Document usepushurl in the man page

usepushurl is a great setting - it reduces the number of actions git has
to do for things like a bare 'git fetch'. It is, unfortunately, not
documented in the manpage.

Change-Id: Ia7b179c6fcf6490454087aa359d891d1ff3af1a0

openstack-infra/git-review