Give proper error message when refusing to overwrite existing branch
If fast-export was asked to export a Mercurial branch to Git and a
branch of the same name already existed in the Git repo but it was not
created by fast export, fast-export would crash while trying to format
an error message claiming that the destination branch was modified
behind its back.
This patch extends fast-export to detect the situation above and give
a proper error message which hopefully is less confusing to the user.
Credits for discovering the original crash goes to Shun-ichi Goto
<gotoh@taiyo.co.jp>.
Closes: #269.