Support "pull from one place, push to another place" workflow
better by introducing remote.pushdefault (overrides the "origin"
thing) and branch.*.pushremote (overrides the branch.*.remote).
* rr/triangle:
remote.c: introduce branch.<name>.pushremote
remote.c: introduce remote.pushdefault
remote.c: introduce a way to have different remotes for fetch/push
t5516 (fetch-push): drop implicit arguments from helper functions
t5516 (fetch-push): update test description
remote.c: simplify a bit of code using git_config_string()