Allow build on systems without git (#2514)

Set git revision to "unknown" if git not available.

app/build.gradle

@@ -7,10 +7,14 @@ apply from: "../instance-build.gradle"
 
 def getGitSha = {
     def stdout = new ByteArrayOutputStream()
+    try {
         exec {
             commandLine 'git', 'rev-parse', '--short', 'HEAD'
             standardOutput = stdout
         }
+    } catch (Exception e) {
+        return "unknown"
+    }
     return stdout.toString().trim()
 }