[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