Add a "--base" option to the "git flow {bugfix,feature} rebase" command

[jkankiewicz]

Please add a --base <base> command line option to the git flow {bugfix,feature} rebase command so that an orphaned bugfix/feature branch can be conveniently rebased on develop—or another bugfix/feature branch—while also updating its corresponding section in the .git/config file to record its new base branch.

Ideally every bugfix/feature branch should be based upon the develop branch but the git flow {bugfix,feature} start <name> command does support an optional <base> argument for when it shouldn't.
I use that optional argument to start a new bugfix/feature branch that is based upon a prior bugfix/feature branch whose changes are still under review so that I'm not blocked on that branch being finished.
Of course, when that prior bugfix/feature branch is finished I then need to rebase its orphaned child bugfix/feature branch on the develop branch but the git flow {bugfix,feature} rebase command doesn't provide a way to do it because its implicit <base_branch> argument can only be read from the .git/config file, it cannot be specified explicitly on the command line.
For example,

$ git flow feature start parent
Switched to a new branch 'feature/parent'

Summary of actions:
- A new branch 'feature/parent' was created, based on 'develop'
- You are now on branch 'feature/parent'

Now, start committing on your feature. When done, use:

     git flow feature finish parent

$ git flow feature start child feature/parent
Switched to a new branch 'feature/child'

Summary of actions:
- A new branch 'feature/child' was created, based on 'feature/parent'
- You are now on branch 'feature/child'

Now, start committing on your feature. When done, use:

     git flow feature finish child

$ git flow feature finish parent
Will try to rebase 'parent' which is based on 'develop'...
Successfully rebased and updated refs/heads/feature/parent.
Switched to branch 'develop'
Your branch is up-to-date with 'origin/develop'.
Already up-to-date.
Deleted branch feature/parent (was ecb52f56).
$ cat .git/config
...
[gitflow "branch.feature/child"]
	base = feature/parent
...
$ git flow feature finish child
Fatal: The base 'feature/parent' doesn't exists locally or is not a branch. Can't finish the feature branch 'feature/child'.

While I could rebase an orphaned bugfix/feature branch using the git rebase command directly, doing so will not update the bugfix/feature branch's corresponding section in the .git/config file; it will continue to reference the non-existent bugfix/feature branch that it was originally based upon when it was started so I'd have to also edit the bugfix/feature branch's corresponding section in the .git/config file manually which places its referential integrity at further risk of human error.