Main: A<-B
      ^
      |
Fork: C

Merge 1:

Main: A<-B
      ^  ^
      |  |
Fork: C<-D

Merge base for D is A.

More work happens in each branch:

Main: A<-B<-E<-F
      ^  ^     ^
      |  |     |
Fork: C<-D<-G<-H

Is the merge base for H still A?