[fx] Add fx make-integration-patch

Sample output:
INSTRUCTIONS:

For each change you wish to test with global integration:
  1) Navigate to the change in Gerrit's web UI
  2) Click on Download Patch (in the menu at the top right)
  3) Copy any of the download options
  4) Paste it here followed by a newline

When all changes have been added, type ^D

... GO!

git fetch "https://fuchsia.googlesource.com/fuchsia" refs/changes/08/272008/11 && git checkout FETCH_HEAD
git fetch "https://fuchsia.googlesource.com/fuchsia" refs/changes/08/272008/11 && git checkout FETCH_HEAD

I'm about to write /usr/local/google/home/thatguy/src/fuchsia/integration/patches.json with the following contents
and then upload the patch file to Gerrit:

[
  {
    "ref": "refs/changes/68/275868/2",
    "host": "fuchsia.googlesource.com",
    "project": "fuchsia"
  },
  {
    "ref": "refs/changes/68/275868/2",
    "host": "fuchsia.googlesource.com",
    "project": "fuchsia"
  }
]

Do you want to continue? [yN] y
<snip>

Change-Id: I1eb2ad78f492eeb75eea0fb997f0e0e930a35d6d

tools/devshell/contrib/OWNERS

@@ -22,3 +22,4 @@ per-file save-package-stats=raggi@google.com
 per-file set-petal=abarth@google.com
 per-file set-relay=tamird@google.com
 per-file update-rustc-third-party=cramertj@google.com
+per-file make-integration-patch=thatguy@google.com

tools/devshell/contrib/make-integration-patch

+#!/bin/bash
+# Copyright 2019 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+### Creates a CL in an integration repository suitable for testing other CLs
+### using the global integration commit queue.
+
+## usage: fx make-integration-cl
+##
+## The command prints instructions and prompts the user for input. Once
+## all input has been collected, it creates a patches.json file in
+## integration/, creates a changelist and pushes it to Gerrit.
+
+source "$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"/../lib/vars.sh || exit $?
+fx-config-read
+
+function print-instructions {
+  echo "INSTRUCTIONS:"
+  echo
+  echo "For each change you wish to test with global integration:"
+  echo "  1) Navigate to the change in Gerrit's web UI"
+  echo "  2) Click on Download Patch (in the menu at the top right)"
+  echo "  3) Copy any of the download options"
+  echo "  4) Paste it here followed by a newline"
+  echo
+  echo "When all changes have been added, type ^D"
+  echo
+  echo "... GO!"
+  echo
+}
+
+function print-after-instructions {
+  echo
+  echo "The URL for your change is above. You can now navigate there and"
+  echo "CQ+1 (dry run) the CL to test global integration with your patches."
+}
+
+function abort-if-unclean {
+  local dir="$1"
+  if ! git -C "${dir}" status --porcelain; then
+    echo "ERROR: git repo at ${dir} must be clean." > /dev/stderr
+    exit 1
+  fi
+}
+
+function write-patches-json {
+  local file="$1/patches.json"
+  local contents="$2"
+
+  echo "$contents" > $file
+}
+
+function prompt-or-exit {
+  local prompt="$1"
+  read -p "${prompt} [yN] " -n 1 -r yesno
+  echo
+  if [[ ! $yesno =~ ^[Yy]$ ]]; then
+    exit 0
+  fi
+}
+
+function create-git-commit-and-upload {
+  local dir="$1"
+  git -C $dir add -A
+  git -C $dir commit -m "DO NOT SUBMIT - testing unsubmitted changes in GI"
+  git -C $dir push origin HEAD:refs/for/master
+  return $?
+}
+
+function main {
+  local integration_dir="${FUCHSIA_DIR}/integration"
+
+  abort-if-unclean $integration_dir
+  print-instructions
+
+  patches=$(
+    local line host project ref first=true
+    while read line; do
+      local url=$(echo "$line" | cut -d " " -f 3 | sed 's/"//g')
+      host=$(echo "$url" | cut -d / -f 3)
+      project=$(echo "$url" | cut -d / -f 4)
+      ref=$(echo "$line" | cut -d " " -f 4)
+
+      if [ $first = false ]; then
+        printf ',\n'
+      fi
+      first=false
+      printf '  {\n    "ref": "%s",\n    "host": "%s",\n    "project": "%s"\n  }' "$ref" "$host" "$project"
+    done < /dev/stdin
+  )
+
+  local patch_file_contents=$(printf '[\n%s\n]' "$patches")
+
+  echo
+  echo "I'm about to write ${integration_dir}/patches.json with the following contents"
+  echo "and then upload the patch file to Gerrit:"
+  echo
+  echo "$patch_file_contents"
+  echo
+
+  prompt-or-exit "Do you want to continue?"
+  write-patches-json "$integration_dir" "$patch_file_contents"
+  if create-git-commit-and-upload "$integration_dir"; then
+    print-after-instructions
+  else
+    echo
+    echo "ERROR: could not push your patch change to Gerrit. See details above."
+    echo "Once you have resolve the error, you can push the change to Gerrit "
+    echo "manually by invoking: "
+    echo
+    echo "git -C integration push origin HEAD:refs/for/master"
+  fi
+}
+
+main "$@"