[zprezto-update] Add convenience function to update zprezto (#1344)

* [zprezto-update] Add convenience function to update zprezto

This function checks if there is any update to zprezto, and if so
will pull in the changes. It will not attempt a pull unless
it is fastforwardable. It also makes sure the user is on the master
branch before attempting.

* [zprezto-update] Improve resilience of the function

Better error checking of status of the git repository
and better error producing.

Fit columns into mostly 80 width and add a missing printf
argument.

Use ( ) around the function so changing directory does not affect
the outer scope.

* [README] Add instructions on using zprezto-update function

init.zsh

@@ -17,6 +17,52 @@ if ! autoload -Uz is-at-least || ! is-at-least "$min_zsh_version"; then
 fi
 unset min_zsh_version
 
+# zprezto convenience updater
+# The function is surrounded by ( ) instead of { } so it starts in a subshell
+# and won't affect the environment of the calling shell
+function zprezto-update () (
+  function cannot-fast-forward {
+    local STATUS="$1"
+    [[ "${STATUS}" ]] && printf "%s\n" "${STATUS}"
+    printf "Unable to fast-forward the changes. You can fix this by "
+    printf "running\ncd '%s' and then\n'git pull' " "${ZPREZTODIR}"
+    printf "to manually pull and possibly merge in changes\n"
+  }
+  cd -q -- "${ZPREZTODIR}" || return 7
+  local orig_branch="$(git symbolic-ref HEAD 2>/dev/null | cut -d '/' -f 3)"
+  if [[ "$orig_branch" == "master" ]]; then
+    git fetch || return "$?"
+    local UPSTREAM=$(git rev-parse '@{u}')
+    local LOCAL=$(git rev-parse @)
+    local REMOTE=$(git rev-parse "$UPSTREAM")
+    local BASE=$(git merge-base @ "$UPSTREAM")
+    if [[ $LOCAL == $REMOTE ]]; then
+      printf "There are no updates.\n"
+      return 0
+    elif [[ $LOCAL == $BASE ]]; then
+      printf "There is an update availible. Trying to pull.\n\n"
+      if git pull --ff-only; then
+        printf "Syncing submodules\n"
+        git submodule update --recursive
+        return $?
+      else
+        cannot-fast-forward
+        return 1
+      fi
+    elif [[ $REMOTE == $BASE ]]; then
+      cannot-fast-forward "Commits in master that aren't in upstream."
+      return 1
+    else
+      cannot-fast-forward "Upstream and local have diverged."
+      return 1
+    fi
+  else
+    printf "zprezto install at '%s' is not on the master branch " "${ZPREZTODIR}"
+    printf "(you're on '%s')\nUnable to automatically update.\n" "${orig_branch}"
+    return 1
+  fi
+  return 1
+)
 #
 # Module Loader
 #