git: Set push.default

Do not push anything (error out) unless a refspec is given. We want
to avoid mistakes by always being explicit.

See https://git-scm.com/docs/git-config#Documentation/git-config.txt-pushdefault

git/.gitconfig

@@ -36,6 +36,7 @@
 	rebase = false
 [push]
 	followTags = true
+	default = nothing
 [interactive]
 	diffFilter = delta --color-only
 [delta]