git: setup tracking branches via hooks

We want all branches to track origin/HEAD by default but autoSetupMerge
only allows inherit which will work for new local branches (assuming our
start point has tracking setup correctly). The other options will
auto-track the remote branch which isn't what we want.

.config/git/config

@@ -5,7 +5,7 @@
 	conflictstyle = diff3
 [core]
 	autocrlf = input
-	hooksPath = /dev/null
+	hooksPath = ~/.config/git/hooks
 [delta]
 	features = side-by-side line-numbers decorations
 	whitespace-error-style = 22 reverse
@@ -24,7 +24,8 @@
 [push]
 	default = current
 [branch]
-	autoSetupMerge = inherit
+	# we use a hook to set this up instead
+	autoSetupMerge = false
 [remote]
 	pushDefault = origin
 [alias]

.config/git/hooks/post-checkout

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+# ignore non-branch checkouts
+if [[ "$3" != "1" ]]; then
+  exit
+fi
+
+# skip if upstream is already set
+if git rev-parse --verify --quiet '@{u}' > /dev/null 2>&1; then
+  exit
+fi
+
+# use upstream/HEAD if it exists, otherwise origin/HEAD
+if git show-ref --verify --quiet refs/remotes/upstream/HEAD; then
+  git branch --set-upstream-to refs/remotes/upstream/HEAD
+else
+  git branch --set-upstream-to refs/remotes/origin/HEAD
+fi