[PATCH/RFC] fast-import: disallow empty branches as parents

Combinations of "reset", "commit" with "from" and/or "merge" commands may make fast-import to produce bad objects (null_sha1 parents) or accept bad inputs (ones asking for empty branches as parents).

Fix this and add some tests.

One RFC here: does following use case make any sense/should it be allowed?

    commit refs/heads/master
    ...
    from something
    merge refs/heads/master

1. If "from" is omitted or equals refs/heads/master we end up with duplicated parents.
2. And if something is not master we allow to pick a new first parent path.

"2" seems quite legal, while "1" looks worse. Though "1" is not directly related to this patch and can be reproduced via a simple "merge X X" command for example.

Dmitry Ivankov (1):
  fast-import: disallow empty branches as parents

 fast-import.c          |   49 +++++++++++++++++++++++++++++------------------
 t/t9300-fast-import.sh |   48 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+), 19 deletions(-)