Do not set commit parent if branch is dirty.
Do not set commit parent if branch is dirty.

Having commit parent tells svn-fast-export to emit fast-import's
"from" command for a new commit. In case of dirty branch it triggered
incorrect root initialization with previous branch tree state, although
that branch was removed previously.

satori/git-svn-fast-import,satori/git-svn-fast-import,satori/git-svn-fast-import