Add git co-upstream-pr command (#726)

* Add git co-upstream-pr

When reviewing Pull Requests from open source projects, it's handy
to have a way to just pass the PR number and the local branch name.

With this new command, to fetch a PR from an upstream remote repo,
we just need the PR number and the name of the local branch:

$ co-upstream-pr 001 test_generator_pr

git will checkout the upstream PR in the local test_generator_pr branch.

If there is no upstream remote set, we get a nice message guiding
users to set it first.

Co-authored-by: Mina Slater <[email protected]>

* Use heredoc to send output into cat instead of using echo and multi-line quotes

* Subcommand, not alias

* Fail gracefully and exit if PR number or local branch name are not provided

Co-authored-by: Mike Burns <[email protected]>

---------

Co-authored-by: Mina Slater <[email protected]>
Co-authored-by: Mike Burns <[email protected]>

README.md

@@ -198,6 +198,7 @@ configuration:
 
 [git](http://git-scm.com/) configuration:
 
+- Adds a `co-upstream-pr $PR_NUMBER $LOCAL_BRANCH_NAME` subcommand to checkout remote upstream branch into a local branch.
 - Adds a `create-branch` alias to create feature branches.
 - Adds a `delete-branch` alias to delete feature branches.
 - Adds a `merge-branch` alias to merge feature branches into master.

bin/git-co-upstream-pr

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+set -e
+
+pull_request_number=$1
+local_branch_name=$2
+
+if [ -z "$pull_request_number" -o -z "$local_branch_name" ]; then
+  echo "usage: git co-upstream-pr <pull_request_number> <local_branch_name>"
+  exit 1
+fi
+
+if git remote -v | grep -q upstream; then
+  git fetch upstream "pull/$pull_request_number/head:$local_branch_name"
+  git checkout "$local_branch_name"
+else
+  cat <<HELP
+You don't have an upstream remote set.
+Use:
+  git remote add upstream {upstream_remote_url}
+
+to set the reference and then try again.
+HELP
+fi