Merge branch 'maint' of git://repo.or.cz/git/fastimport into maint

* 'maint' of git://repo.or.cz/git/fastimport:
Fix possible coredump with fast-import --import-marks
Refactor fast-import branch creation from existing commit
fast-import: Fix crash when referencing already existing objects
fast-import: Fix uninitialized variable