Accidentally branched off of the wrong branch, and when I want to merge into the master I have to merge both branches

Try git rebase –onto with the following syntax:

To put branch2’s changes on to the master without including branch1’s

git rebase --onto master branch1 branch2

Relevant output from git help rebase:

Here is how you would transplant a topic branch based on one branch to another, to
pretend that you forked the topic branch from the latter branch, using rebase --onto.

First let’s assume your topic is based on branch next. For example, a feature
developed in topic depends on some functionality which is found in next.

              o---o---o---o---o  master
                   \
                    o---o---o---o---o  next
                                     \
                                      o---o---o  topic

We want to make topic forked from branch master; for example, because the functionality
on which topic depends was merged into the more stable master branch.
We want our tree to look like this:

              o---o---o---o---o  master
                  |            \
                  |             o'--o'--o'  topic
                   \
                    o---o---o---o---o  next

We can get this using the following command:

git rebase --onto master next topic

Keep in mind the risks and pitfalls of rebasing, mentioned here:
http://git-scm.com/book/en/Git-Branching-Rebasing