project: rev_list: simplify execution

Currently we read the binary stream from the subprocess code directly
before waiting for it to finish, but there's no need to do so as we
aren't streaming the output to the user.  This also means we pass up
binary data to the caller as we don't go through GitCommand's internal
logic which decodes the stream as utf-8.

Simplify the code by calling Wait first, then splitting the entire
captured output in one line.

Bug: https://crbug.com/gerrit/10418
Change-Id: I7a57904be8cb546a229980fb79c829fc3df31e7d