update with no arguments should follow remote, not upstream

[zombiezen]

Steps to Reproduce

# On an existing repository with master
$ gg branch foo  # implicit upstream of origin/master
$ gg push -create myfork
# changes made on myfork
$ gg pull -r foo myfork  # I realize I could have used -u here, but I forgot.
$ gg update

Expected Behavior

A fast-forward merge to myfork/foo.

Actual Behavior

gg up attempts to fast-forward merge on upstream, which results in exiting without doing anything with this error message:

fatal: Not possible to fast-forward, aborting.
gg: run git merge: exit status 128

Version Info

gg version 0.6.1, built on 2018-08-22T22:18:40Z
go: go1.10.3 gc linux/amd64
git version 2.19.1

[zombiezen]

I believe the logic I want is:

• If the upstream has the same branch name as the current branch (i.e. if config branch.CURRENT.merge == CURRENT, where CURRENT is the HEAD ref), then use that to update.
• Otherwise, use the ref from the default push remote.

Both of these require then looking up the remote.*.fetch configuration value and mapping the ref to a remote-tracking branch.