Use number of commits on branch as the version code

[oliverlockwood]

Connects to #179.

The Play store APIs only accept an APK submission if the versionCode (an integer) has not already been used. You can see an example failure here:

:cyclestreets.app:publishApkRelease FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':cyclestreets.app:publishApkRelease'.
> com.google.api.client.googleapis.json.GoogleJsonResponseException: 403 Forbidden
 {
   "code" : 403,
   "errors" : [ {
     "domain" : "androidpublisher",
     "message" : "APK specifies a version code that has already been used.",
     "reason" : "apkUpgradeVersionConflict"
   } ],
   "message" : "APK specifies a version code that has already been used."
 }

Therefore, versionCode has to be monotonically increasing.

The most logical way I could come up with for deriving a monotonically increasing number for our CI builds was to use the number of commits. On the master branch (which is the only branch that will attempt to push to the Play store anyway), this number should never go down, assuming that we don't intend to rewrite history.

[mvl22]

Great idea - thanks.

[oliverlockwood]

👍