git bisect Vs branch

Considering the following story what is the method to find the regression with bisect?

I cloned a git repository (origin) which derives from another one (first-origin). A merge is done from first-origin to origin at each stable release (identified by a tag).

first-origin/master  *---A---------B-----------------------C-
                          \         \                       \     
origin/master              ----------B'----------U-----------C'-
                                      \           \           \   
master                                 ------------U'----------C''-

Now, after that I merged C' I fixed the conflicts and compiled without error but I have a regression. It could come from any commit between B and C or U and C', and I need to modify my code to correct the issue.

I would like to find the commit which introduce this regression by using git bisect but as the history is not linear it is not so easy (1). It though to create a linear history but I have no idea how to proceed...

Thanks, Grégory

(1) http://www.kernel.org/pub/software/scm/git/docs/user-manual.html#bisect-merges