Adds script to do release setup. (#481)

scripts/prepare_release.sh

@@ -0,0 +1,76 @@
+#!/bin/bash -
+# Usage: ./prepare_release.sh <release version>
+
+set -e
+
+Colorize() {
+	echo "$(tput setff $2)$1$(tput sgr0)"
+}
+
+EchoRed() {
+	echo "$(tput setaf 1; tput bold)$1$(tput sgr0)"
+}
+EchoGreen() {
+	echo "$(tput setaf 2; tput bold)$1$(tput sgr0)"
+}
+
+Die() {
+	EchoRed "$1"
+	exit 1
+}
+
+DieUsage() {
+    Die "Usage: ./prepare_release.sh <release version>"
+}
+
+# Usage: CheckVersion <version>
+CheckVersion() {
+    [[ $1 =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]] || Die "Version not in ###.###.### format."
+}
+
+# Usage: IncrementVersion <version>
+IncrementVersion() {
+    local version=$1
+    local minorVersion=$(echo $version | sed 's/[0-9][0-9]*\.[0-9][0-9]*\.\([0-9][0-9]\)*/\1/')
+    local nextMinorVersion=$((minorVersion+1))
+    echo $version | sed "s/\([0-9][0-9]*\.[0-9][0-9]*\)\.[0-9][0-9]*/\1.$nextMinorVersion/"
+}
+
+[ $# -ne 2 ] || DieUsage
+
+EchoGreen '===== RELEASE SETUP SCRIPT ====='
+
+VERSION=$1
+CheckVersion ${VERSION}
+
+NEXT_VERSION=$(IncrementVersion $VERSION)
+CheckVersion ${NEXT_VERSION}
+
+if [[ $(git status -uno --porcelain) ]]; then
+    Die 'There are uncommitted changes.'
+fi
+
+# Checks out a new branch for this version release (eg. 1.5.7).
+git checkout -b ${VERSION}
+
+# Updates the pom.xml with the version to release.
+mvn versions:set versions:commit -DnewVersion=${VERSION}
+
+# Tags a new commit for this release.
+git commit -am "preparing release ${VERSION}"
+git tag v${VERSION}
+
+# Updates the pom.xml with the next snapshot version.
+# For example, when releasing 1.5.7, the next snapshot version would be 1.5.8-SNAPSHOT.
+NEXT_SNAPSHOT=${NEXT_VERSION}-SNAPSHOT
+mvn versions:set versions:commit -DnewVersion=${NEXT_SNAPSHOT}
+
+# Commits this next snapshot version.
+git commit -am "${NEXT_SNAPSHOT}"
+
+# Pushes the release branch to Github.
+git push --tags --set-upstream origin ${VERSION}
+
+# File a PR on Github for the new branch. Have someone LGTM it, which gives you permission to continue.
+EchoGreen 'File a PR for the new release branch:'
+echo https://github.com/GoogleCloudPlatform/appengine-plugins-core/compare/${VERSION}