git history in a file

Hello,

I'm trying to figure out how to generate a file that contains the git
history and changes from one revision to another, such that when the
file is applied to a different tree starting at the same ancestor, the
resulting tree is identical.

E.g. git <whatever> v4.2-rc3..master

would produce this file, and then:

git <apply whatever> <file>

run on a tree which is currently at 4.2-rc3 would result in an identical HEAD.

Originally I thought git-format-patch would be sufficient, however it
does not contain merge commits and the resulting commit shas seem like
they would be inaccurate.  Is there a way to do this?

I realize one could just git fetch/pull from tree A to tree B, but the
usecase here is more complicated.  Please CC me on replies as I am not
subscribed.

josh