Add more checks to prevent build failures in gradle branch suffix

- Add function `getGitWorkingBranch` that returns the current working branch, and "" if it could not be determined (either because git is not installed or because the directory is not a git repo).
- Make sure normalizedWorkingBranch is not empty (leading to an invalid app id terminating with `.`)
- Make normalizedWorkingBranch lowercase
- Add comments

app/build.gradle

@@ -3,6 +3,24 @@ apply plugin: 'kotlin-android'
 apply plugin: 'kotlin-android-extensions'
 apply plugin: 'kotlin-kapt'
 
+
+static String getGitWorkingBranch() {
+    try {
+        def gitProcess = "git rev-parse --abbrev-ref HEAD".execute()
+        gitProcess.waitFor()
+        if (gitProcess.exitValue() == 0) {
+            return gitProcess.text.trim()
+        } else {
+            // not a git repository
+            return ""
+        }
+    } catch (IOException ignored) {
+        // git was not found
+        return ""
+    }
+}
+
+
 android {
     compileSdkVersion 28
     buildToolsVersion '28.0.3'
@@ -31,12 +49,14 @@ android {
             debuggable true
 
             // suffix the app id and the app name with git branch name
-            def workingBranch = "git rev-parse --abbrev-ref HEAD".execute().text.trim()
+            def workingBranch = getGitWorkingBranch()
-            if (workingBranch.isEmpty() || workingBranch == "master" || workingBranch == "dev") {
+            def normalizedWorkingBranch = workingBranch.replaceAll("[^A-Za-z]+", "").toLowerCase()
+            if (normalizedWorkingBranch.isEmpty() || workingBranch == "master" || workingBranch == "dev") {
+                // default values when branch name could not be determined or is master or dev
                 applicationIdSuffix ".debug"
                 resValue "string", "app_name", "NewPipe Debug"
             } else {
-                applicationIdSuffix ".debug." + workingBranch.replaceAll("[^A-Za-z]+", "")
+                applicationIdSuffix ".debug." + normalizedWorkingBranch
                 resValue "string", "app_name", "NewPipe " + workingBranch
             }
         }