jruby/heroku-buildpack-jruby has trouble when Gemfile contains a git gem

heroku-build.log

-----> Heroku receiving push
-----> Fetching custom buildpack... done
-----> JRuby app detected
-----> Downloading and unpacking JRuby
-----> Installing JRuby-OpenSSL, Bundler and Rake
       Successfully installed bouncy-castle-java-1.5.0146.1
       Successfully installed jruby-openssl-0.7.6.1
       Successfully installed bundler-1.1.3
       Successfully installed rake-0.9.2.2
       4 gems installed
-----> Vendoring JRuby into slug
-----> Installing dependencies with Bundler
       Fetching gem metadata from http://rubygems.org/........
       Fetching git://github.com/padrino/padrino-framework.git
       Installing rake (0.9.2.2) 
       Installing i18n (0.6.0) 
       Installing multi_json (1.3.5) 
       Installing activesupport (3.2.3) 
       Installing bouncy-castle-java (1.5.0146.1) 
       Installing haml (3.1.5) 
       Installing rack (1.4.1) 
       Installing url_mount (0.2.1) 
       Installing http_router (0.10.2) 
       Installing jruby-openssl (0.7.6.1) 
       Installing jruby-rack (1.1.6) 
       Installing mime-types (1.18) 
       Installing polyglot (0.3.3) 
       Installing treetop (1.4.10) 
       Installing mail (2.3.3) 
       Installing rack-protection (1.2.0) 
       Installing tilt (1.3.3) 
       Installing sinatra (1.3.2) 
       Installing thor (0.14.6) 
       Using padrino-core (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using padrino-helpers (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using padrino-admin (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using padrino-cache (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using bundler (1.1.3) 
       Using padrino-gen (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using padrino-mailer (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Using padrino (0.10.6) from git://github.com/padrino/padrino-framework.git (at master) 
       Installing rdiscount (1.6.8) with native extensions 
       Installing sinatra-flash (0.3.0) 
       Installing trinidad_jars (1.0.3) 
       Installing trinidad (1.3.5) 
       Your bundle is complete! It was installed into ./vendor/bundle
       Dependencies installed
cp: cannot create regular file `/app/tmp/repo.git/.cache/vendor/bundle/ruby/1.9.1/cache/bundler/git/padrino-framework-da42868283fca4941ccdcfc887a5cde85a7e730a/objects/pack/pack-045d2c2658496bb2c386678a62f1b5db0b75967c.idx': Permission denied
cp: cannot create regular file `/app/tmp/repo.git/.cache/vendor/bundle/ruby/1.9.1/cache/bundler/git/padrino-framework-da42868283fca4941ccdcfc887a5cde85a7e730a/objects/pack/pack-045d2c2658496bb2c386678a62f1b5db0b75967c.pack': Permission denied
cp: cannot create regular file `/app/tmp/repo.git/.cache/vendor/bundle/ruby/1.9.1/bundler/gems/padrino-framework-78329337e6bc/.git/objects/pack/pack-045d2c2658496bb2c386678a62f1b5db0b75967c.idx': Permission denied
cp: cannot create regular file `/app/tmp/repo.git/.cache/vendor/bundle/ruby/1.9.1/bundler/gems/padrino-framework-78329337e6bc/.git/objects/pack/pack-045d2c2658496bb2c386678a62f1b5db0b75967c.pack': Permission denied
 !     Heroku push rejected, failed to compile Jruby app