Chanot branch[0].isdigit()nging --milestone to use JSON source and adding prompt for branch mismatch.

Sample output:

$ drover --merge 196360 --milestone 27

Not all platforms have same branch number for M27.

Here's a list of platforms on each branch:
-(1453): win, ios, cf, mac, linux, android
-(1460): cros
Which branch? ('q' to cancel) 1453

Review URL: https://chromiumcodereview.appspot.com/14544004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@198282 0039d316-1c4b-4281-b951-d872f2087c98