Fix a bug where we would corrupt the stuff read from git-rev-list.
If we have a very long commit message, and we end up getting a
bufferfull of data from git-rev-list that all belongs to one commit,
we ended up throwing away the data from a previous read that should
have been included. The result was a error message about not being
able to parse the output of git-rev-list.
Also, if the git-rev-list output that we can't parse is long, only put
the first 80 chars in the error message. Otherwise we end up with an
enormous error window.