Use `git branch --contains` to check if a commit has been merged.

Fix #66.
Compare upstream/master with feature branch when sending pr to upstream.

Fix #73.
Fix tests for create command.

Fix #69.
Merge pull request #78 from xystushi/review_130913_fix_test_for_prepare_command

Fix tests for create command.
Fixes #69.
Merge pull request #77 from xystushi/review_130913_compare_upstream_with_feature_branch_before_sending_pr

Compare upstream/master with feature branch when sending pr to upstream.
Fixes #73
Merge pull request #76 from xystushi/review_130912_bugfix_for_checking_commit_merge_status

Use `git branch --contains` to check if a commit has been merged.