Before this change, `git-flow feature start` would nag about the working
tree being dirty and disallow continuation. Then, there were two
alternatives:

- stash away your changes manually, create the new branch, restore the
  stash; or
- use `git-flow feature start -f` to force creation

There is absolutely no good reason for git-flow to forbid this creation,
as git allows it already.  Therefore, the -f behaviour is now the default,
without an option to turn that behaviour off.  Git takes care of unsafe
situations again now.

This fix also works when feature branches are created manually, based on remote
(i.e. other developers) feature branches, to work on features together.

`git branch -d` won't remove local feature branches that are set up to track
remote branches, `-D` will.

Fixes ticket #31.

Sorry!

This can be used as a shortcut to "git checkout feature/full-feature-name".
Feature branch prefix names may be used for convenience.

Only test if local branch is behind origin if origin exists.

They were left behind in the main git-flow script, since they are moved
over to gitflow-common.

new repo is created.

``prefix''. There is no need to install git-flow inside git's libexec dir by
default. It just needs to be *somewhere* on the PATH.

functions, then gitflow-specific functions and finally, assertions to use
in gitflow subcommands.

git specific and git-flow specific functions:

gitflow_current_branch             -> git_current_branch
gitflow_is_branch_merged_into      -> git_is_branch_merged_into
gitflow_local_branch_exists        -> git_local_branch_exists
gitflow_local_branches             -> git_local_branches
gitflow_remote_branches            -> git_remote_branches
gitflow_require_branch             -> require_branch
gitflow_require_branch_absent      -> require_branch_absent
gitflow_require_branches_equal     -> require_branches_equal
gitflow_require_clean_working_tree -> require_clean_working_tree
gitflow_require_git_repo           -> require_git_repo
gitflow_require_git_repo           -> require_git_repo
gitflow_require_initialized        -> require_gitflow_initialized
gitflow_require_initialized        -> require_gitflow_initialized
gitflow_require_local_branch       -> require_local_branch
gitflow_require_remote_branch      -> require_remote_branch
gitflow_require_tag_absent         -> require_tag_absent
gitflow_tag_exists                 -> git_tag_exists
gitflow_test_branches_equal        -> git_compare_branches
gitflow_test_clean_working_tree    -> git_is_clean_working_tree
resolve_nameprefix                 -> gitflow_resolve_nameprefix

GIT_EXEC_PATH overrideable via Makefile command arguments.