push: suggest remote.pushDefault when "simple" refuses to push - #2467

After cloning a project and adding a fork as a second remote, a
branch started from "origin/main" cannot be pushed with a plain
"git push" while push.default is left at "simple", because its
upstream branch has a different name. The advice offers to push to
the upstream branch, or to a branch of the same name on "origin".
Both go to the project's repository, which someone working in a fork
usually cannot push to. The usual way out, "git push -u" to the fork,
moves the upstream there, so the branch is no longer compared with
the project's branch.

Setting remote.pushDefault to the fork makes a plain "git push" go to
a branch of the same name there, and leaves the upstream alone. When
the repository has another remote, show how to push to it, and how
to make that the default with remote.pushDefault. With more than one
other remote, show "<remote>" in place of a name.

Show it in place of the advice to set branch.autoSetupMerge to
"simple". That setting would leave a branch started from
"origin/main" without an upstream, and with it the comparison with
the project's branch.

Do not show it when remote.pushDefault or branch.<name>.pushRemote
is already set. The push then went to this remote on purpose, as in a
trunk-based setup where every branch is pushed to the one remote.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>