feat: Prefer "upstream" push strategy by default

Makes it easier to see what's the status of the topic branch in relation
to the upstream.

Highly recommended by tpope
tpope/vim-fugitive#1700 (comment)

Simple has been default since Git 2.0 and is good for beginners.

  upstream - push the current branch back to the branch whose changes
  are usually integrated into the current branch (which is called
  @{upstream}). This mode only makes sense if you are pushing to the
  same repository you would normally pull from (i.e. central workflow).

softlinks/.gitconfig

@@ -16,7 +16,7 @@
 	verbose = true
 	template = ~/.gitmessage
 [push]
-	default = simple
+	default = upstream
 	autoSetupRemote = true
 [pull]
 	rebase = true