Merge branch 'yk/filter-branch-non-committish-refs' into next

when refs that do not point at committish are given, "git
filter-branch" gave a misleading error messages.  This has been
corrected.

* yk/filter-branch-non-committish-refs:
filter-branch: fix errors caused by refs that point at non-committish

diff --cc git-filter-branch.sh
Simple merge