With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.
Let's start by letting the user override the refspecs for download.
The current FETCH_HEAD parsing code assumes that a quote must end the
branch name. Git however allows for quotes as part of a branch name,
which causes us to consider the FETCH_HEAD file as invalid.
Instead of searching for a single quote char, search for a quote char
followed by SP, which is not a valid part of a ref name.
Cloning from an empty repo must set master's upstream to origin's
master, even if neither of them exist.
Fetching from a non-empty origin must then mark the master branch
for-merge. This currently fails.
