Heroku build log showing problem with determining git revision

build.log

-----> Using set buildpack heroku/java
-----> Java app detected
-----> Installing OpenJDK 1.8... done
-----> Executing: mvn -B -DskipTests clean dependency:list install
       [INFO] Scanning for projects...
       [INFO]                                                                         
       [INFO] ------------------------------------------------------------------------
       [INFO] Building spring-boot-heroku-demo 0.2.0
       [INFO] ------------------------------------------------------------------------
       [INFO] 
       [INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ spring-boot-heroku-demo ---
       [INFO] 
       [INFO] --- maven-dependency-plugin:2.9:list (default-cli) @ spring-boot-heroku-demo ---
       [INFO] 
       [INFO] --- buildnumber-maven-plugin:1.4:create (default) @ spring-boot-heroku-demo ---
       [INFO] Executing: /bin/sh -c cd '/tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d' && 'git' 'rev-parse' '--verify' 'HEAD'
       [INFO] Working directory: /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d
       [INFO] Storing buildNumber: null at timestamp: 1450623519645
       [WARNING] Cannot get the branch information from the git repository: 
       Detecting the current branch failed: fatal: Not a git repository (or any parent up to mount point /tmp)
       Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
       
       [INFO] Executing: /bin/sh -c cd '/tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d' && 'git' 'rev-parse' '--verify' 'HEAD'
       [INFO] Working directory: /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d
       [INFO] Storing buildScmBranch: UNKNOWN_BRANCH
       [INFO] 
       [INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ spring-boot-heroku-demo ---
       [INFO] Using 'UTF-8' encoding to copy filtered resources.
       [INFO] Copying 1 resource
       [INFO] Copying 2 resources
       [INFO] 
       [INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ spring-boot-heroku-demo ---
       [INFO] Changes detected - recompiling the module!
       [INFO] Compiling 4 source files to /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d/target/classes
       [INFO] 
       [INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ spring-boot-heroku-demo ---
       [INFO] Using 'UTF-8' encoding to copy filtered resources.
       [INFO] skip non existing resourceDirectory /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d/src/test/resources
       [INFO] 
       [INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ spring-boot-heroku-demo ---
       [INFO] No sources to compile
       [INFO] 
       [INFO] --- maven-surefire-plugin:2.17:test (default-test) @ spring-boot-heroku-demo ---
       [INFO] Tests are skipped.
       [INFO] 
       [INFO] --- maven-jar-plugin:2.5:jar (default-jar) @ spring-boot-heroku-demo ---
       [INFO] Building jar: /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d/target/app.jar
       [INFO] 
       [INFO] --- spring-boot-maven-plugin:1.2.6.RELEASE:repackage (default) @ spring-boot-heroku-demo ---
       [INFO] 
       [INFO] --- maven-install-plugin:2.5.2:install (default-install) @ spring-boot-heroku-demo ---
       [INFO] Installing /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d/target/app.jar to /app/tmp/cache/.m2/repository/com/github/britter/spring-boot-heroku-demo/0.2.0/spring-boot-heroku-demo-0.2.0.jar
       [INFO] Installing /tmp/build_503aea84b3313c9d0ae8587e46ee355c/britter-spring-boot-heroku-demo-846b48d/pom.xml to /app/tmp/cache/.m2/repository/com/github/britter/spring-boot-heroku-demo/0.2.0/spring-boot-heroku-demo-0.2.0.pom
       [INFO] ------------------------------------------------------------------------
       [INFO] BUILD SUCCESS
       [INFO] ------------------------------------------------------------------------
       [INFO] Total time: 4.600 s
       [INFO] Finished at: 2015-12-20T14:58:41+00:00
       [INFO] Final Memory: 30M/233M
       [INFO] ------------------------------------------------------------------------
-----> Discovering process types
       Procfile declares types -> web
-----> Compressing... done, 72.9MB
-----> Launching... done, v23
       https://spring-boot-heroku-demo.herokuapp.com/ deployed to Heroku