Incremental Build Script (#422)

Similar to `plugin_tools.sh` but doesn't depend on Travis or any other CI.

+use it for Cirrus CI

script/incremental_build.sh

diff --git a/script/incremental_build.sh b/script/incremental_build.sh
new file mode 100755
index 0000000..1cefe65
--- /dev/null
+++ b/script/incremental_build.sh
@@ -0,0 +1,24 @@
+#!/bin/bash
+
+set -ev
+
+BRANCH_NAME=$(git rev-parse --abbrev-ref HEAD)
+
+if [ "${BRANCH_NAME}" = "master" ]; then
+  echo "Running for all packages"
+  pub global run flutter_plugin_tools "$@"
+else
+  # Make sure there is up-to-date master.
+  git fetch origin master
+  BRANCH_BASE_SHA=$(git merge-base --fork-point FETCH_HEAD HEAD)
+  echo "Checking changes from $BRANCH_BASE_SHA..."
+  FLUTTER_CHANGED_GLOBAL=`git diff --name-only $BRANCH_BASE_SHA HEAD | grep -v packages | wc -l`
+  FLUTTER_CHANGED_PACKAGES=`git diff --name-only $BRANCH_BASE_SHA HEAD | grep -o "packages/[^/]*" | sed -e "s/packages\///g" | sort | uniq | paste -s -d, -`
+  if [ "${FLUTTER_CHANGED_PACKAGES}" = "" ] || [ $FLUTTER_CHANGED_GLOBAL -gt 0 ]; then
+    echo "Running for all packages"
+    pub global run flutter_plugin_tools "$@"
+  else
+    echo "Running only for $FLUTTER_CHANGED_PACKAGES"
+    pub global run flutter_plugin_tools "$@" --plugins=$FLUTTER_CHANGED_PACKAGES
+  fi
+fi