Merge branch 'jc/for-each-ref-head-segfault-fix'

Using a %(HEAD) placeholder in "for-each-ref --format=" option
caused the command to segfault when on an unborn branch.

* jc/for-each-ref-head-segfault-fix:
for-each-ref: do not segv with %(HEAD) on an unborn branch

diff --cc ref-filter.c
Simple merge