zsh: replace git-up with better script

slight changes to aanand/git-up#121 (comment)

bin/git-up

@@ -1,17 +1,61 @@
-#!/bin/sh
+#!/bin/zsh
 
-original_branch=$(git branch --show-current)
+# Add the following to your ~/.gitconfig
+#
+#  [alias]
+#	   up = !zsh /path-to-this-script/git-up.sh
+#
+#  Then you can call
+#
+#     `git up`
 
-git fetch --all
-branches=$(git branch | sed 's/^\* //')
+# Remember current branch.
+START_BRANCH=$(git branch --show-current --format='%(refname:short)')
 
-for branch in $branches; do
-  echo "Rebasing branch \"$branch\""
-  git checkout "$branch"
-  git pull --rebase --autostash
-  echo ""
-done
+get_local_branches() {
+  # Gets local, which tracks origin.
+  git branch -vv | grep origin | sed 's/*/ /g' | awk '{print $1}'
+}
+
+get_remote_branches() {
+  git branch -r --format='%(refname:short)' | sed 's/origin\///' | grep -v HEAD
+}
+
+get_deleted_branches() {
+  get_remote_branches | grep -Evf /dev/fd/0 <(get_local_branches) | awk '{print $1}'
+}
+
+# Fetch latest.
+# Updates only the view of remote branches and their commits.
+# Does not fast forward local branches.
+# Does not clone missing local remote branches.
+git fetch --all -p >/dev/null
+
+# Delete pruned local branches.
+for branch in $(get_deleted_branches); do
+  if [ "$branch" = "$START_BRANCH" ]; then
+    printf "\n\nDeleting current branch \"%s\". Stashing changes and switching to master.\n" "$branch"
 
-git checkout "$original_branch"
+    # Go to the root dir in case branch switch removes current working dir.
+    cd "$(git rev-parse --show-toplevel)" || exit 1
+    git stash >/dev/null
+    git checkout master >/dev/null
+    START_BRANCH=master
 
-echo "All branches have been updated."
+    git branch -D "$branch" >/dev/null
+  else
+    git branch -D "$branch"
+  fi
+
+done
+
+# Fast-forward all branches.
+for branch in $(get_local_branches); do
+  upstream=$(git config --get "branch.$branch.merge" | sed 's:refs/heads/::')
+  echo "Updating $branch..."
+  if [ "$branch" = "$START_BRANCH" ]; then
+    git merge --ff-only "origin/$upstream" | grep -v "Already up to date"
+  else
+    git fetch . "origin/$upstream:$branch"
+  fi
+done