add a default for the remote to fetch a pr from

[jldugger]

As discussed in #835, a one-liner to fetch a default remote from config.

Testing:

$ git remote
origin
upstream
$ git config git-extras.pr.remote "upstream"
$ cat .git/config
<snip>
[git-extras "pr"]
	remote = upstream
$ ./bin/git-pr 800
remote: Enumerating objects: 44, done.
remote: Counting objects: 100% (44/44), done.
remote: Total 59 (delta 43), reused 43 (delta 43), pack-reused 15
Unpacking objects: 100% (59/59), 24.49 KiB | 338.00 KiB/s, done.
From github.com:tj/git-extras
 * [new ref]         refs/pull/800/head -> pr/800
Switched to branch 'pr/800'

[jldugger]

Previously used --default which was incompatible with git-extra's declared dependency on git 2.6.1+. That is now resolved.

[spacewander]

@jldugger
Merged. Thanks!