set versionName based on git release tag

This makes it easy to tell which debug build a device is running, since the
versionName now automatically describes the exact commit that was built,
based on `git describe`, e.g.:  0.100-alpha7-33-gc2e8e8a

For release builds, i.e. builds from commits that are tagged as releases,
the versionName will be just the tag name: 0.100-alpha8

closes #664 https://gitlab.com/fdroid/fdroidclient/issues/664

app/build.gradle

@@ -3,6 +3,16 @@ apply plugin: 'witness'
 apply plugin: 'checkstyle'
 apply plugin: 'pmd'
 
+/* gets the version name from the latest Git tag, stripping the leading v off */
+def getVersionName = { ->
+    def stdout = new ByteArrayOutputStream()
+    exec {
+        commandLine 'git', 'describe', '--tags', '--always'
+        standardOutput = stdout
+    }
+    return stdout.toString().trim().substring(1)
+}
+
 repositories {
     jcenter()
 }
@@ -150,6 +160,7 @@ android {
     }
 
     defaultConfig {
+        versionName getVersionName()
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
     }
 

app/src/main/AndroidManifest.xml

@@ -2,9 +2,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="org.fdroid.fdroid"
     android:installLocation="auto"
-    android:versionCode="100007"
-    android:versionName="0.100-alpha7"
-    >
+    android:versionCode="100007">
 
     <uses-sdk
         android:minSdkVersion="8"