conversation: refactor addToHistory

The addToHistory function currently iterates over the list of commits to
add only once. This patch refactors the function so that the message
status computations are done in a separate loop from the rest of the
message handling logic. This will make it easier to fix a bug in the
message status computation in a subsequent patch.

GitLab: #1042
Change-Id: I3fceb10057292f84bc8c5e9d668fed7290ace8f2