Question: How to rebase my changes on a new base (ref)?

[Ilyes Gouta <ilyes.gouta@gmail.com>]

Hi,

I've been tracking a project, via the master branch, customizing
certain aspects and applying my changes into a separate local branch
B. In the mean time, there is this new remote branch
origin/release_2.1, that was created for releasing a certain version.
It happened that I merge entirely that master branch into my local
branch, periodically, in order to get the latest change sets. It also
happens in that projects, that commits from branch C get merged into
the project's origin/master.

I'd like to do the following: Take all the those changes I made
against origin/master, checkout a local branch based on
origin/release_2.1 and apply them on it. This way I can get my
customizations re-based against that particular branch (stream)
instead of origin/master being the parent/upstream.

Is it possible to achieve that? does it involves a kind of git
rebasing (--onto ?). So far, I used exclusively a merge flow to
maintain my private branch B in sync with origin/master and that right
now that code base is more recent than origin/release_2.1.

Here is the structure, in ASCII, of the project: (best viewed with a
proportional font)

origin/master
|
|
|  |  |
|  |  |
|  |  |
+--|--\ (sometimes commits in origin/release_2.1 go into origin/master)
|  |  |
|  |  |
|  |  |
|  |  |
|  |  |
|  +  | (git merge origin/master into my_private_branch)
| /|  |
+  |  |
|  |  |
|  |  |
|  +  | (git merge origin/master into my_private_branch)
| /|  |
+  |  |
|  |  | origin/release_2.1
|  |  /
+--|-/
|  |
|  | my_private_branch (git checkout -b)
|  /
+/
|
|

Regards,

-Ilyes