Merge branch 'dk/reflog-walk-with-non-commit'

"git reflog" incorrectly assumed that all objects that used to be
at the tip of a ref must be commits, which caused it to segfault.

* dk/reflog-walk-with-non-commit:
reflog-walk: don't segfault on non-commit sha1's in the reflog

diff --cc reflog-walk.c
Simple merge