This new subcommand lets you easily work together with peers on features. It's
intended to be extremely simple to pull changes from your co-developers.

This implementation may also be ported to release and/or hotfix branches in the
near future, if we agree on the final implementation details.

Example use
===========
Sharing development of feature branches goes as follows:

Suppose Alice and Bob are both developers on project Foo.  They have local
repos that are up-to-date with `origin`.  Then, Alice starts working on some
feature:

   alice$ git flow feature start newprotocol
   Switched to a new branch 'feature/newprotocol'
   ...

Then, she hacks on the new feature, commits as desired, until the feature's
finished.  She then likes Bob to code-review the feature, so she asks Bob to
pull from her.  (Assuming Bob has a Git remote defined, pointing to Alice's
repo.)

    bob$ git remote
    alice

    bob$ git branch
    * develop
      feature/reader
      master

    bob$ git flow feature pull alice newprotocol
    Created local branch feature/newprotocol based on alice's feature/newprotocol.

    bob$ git branch
      develop
    * feature/newprotocol
      feature/reader
      master

Since the new feature branch is already checked out, Bob can immediately start
peer reviewing the code.  He changes the code as desired and commits each
comment individually, so Alice can later read the Git commit log as the peer
review log.

    bob$ git commit
    [feature/newprotocol 1f6fa95] Forgot return statement.
     1 files changed, 1 insertions(+), 1 deletions(-)
    ...

When he's finished, he tells Alice he's done.  Alice then, in turn pulls in the
peer review commits from Bob, using the same command Bob used to fetch the
changes initially.  (Because feature/newprotocol is still her current branch,
she may omit the explicit 'newprotocols' argument.)

    alice$ git flow feature pull bob
    Pulled bob's changes into feature/newprotocol.

If she disagrees with Bob's comments, she may again commit changes and ask Bob
to do the same again.  This leads to a continuous pull cycle until both parties
agree on the final implementation.  Then, Alice (as the feature owner) finished
the feature.  Bob may discard his feature branch.

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.