如何在git上同步提交

I have a branch A on which I make a commit C1. I fork another branch from A; call it B; and make an commit C2 (changes independent of C1). Now later I go back to A and make some changes to C1. I need to update B now with the latest changes so I do a "git rebase A" on B. This gives me conflicts. Is there an easy way to sync with A. I just want to overwrite C1 on branch B (actually B.C1 should be a mirror of A.C1). Or when rebasing, can I specify something like: "auto resolve conflicts by taking all changes from branch A"I have a branch A on which I make a commit C1.