Revert-style merge/Working tree-only checkout?

Hello,

I am somewhat new to Git and I keep running into having to accomplish a certain task and reading through the documentation I can't seem to find any way of doing this easily.

The problem is when branches diverge and I want to sync a branch to another branch with full overwrite, but maintain history and maintain separate branches.

For example, say there is a branch "master" and I create a branch "b1" from master at some point. After this, there are 5 commits (C1,C2,C3,C4,C5) to master and 17 commits to b1 (let's call them cb1, cb2, cb3, ..., cb17). Say I want to create an 18-th commit to "b1" that makes it identical to the C5 (current) state of master. Essentially a single commit wipe of changes cb1 -> cb17 as well as application of C1->C5. So far I have found one way of accomplishing this, but it is difficult, error prone, slow and I just plain don't like it. I feel like there should be an easier way.

What I currently do:

$ rm -rf * $ git checkout -f master $ tar -cvzf /tmp/master.tar.gz * $ git checkout b1 $ rm -rf * $ tar -xvzf /tmp/master.tar.gz $ git add $ git commit -a $ git merge master

I've considered doing something like the following

$ git checkout b1 $ git revert b1~17..b1 $ git merge master

but it also seems wrong, and requires me to count the submits by hand, which seems silly --> I'm not actually reverting anything. I don't know if this would even work.

Any suggestions on how to accomplish this easier? Some sort of a force-checkout that affects working tree only but not the index?

Thank you,

Yuriy