Merge branch 'tb/ref-filter-empty-modifier' into maint

In the "--format=..." option of the "git for-each-ref" command (and its friends, i.e. the listing mode of "git branch/tag"), "%(atom:)" (e.g. "%(refname:)", "%(body:)" used to error out. Instead, treat them as if the colon and an empty string that follows it were not there. * tb/ref-filter-empty-modifier: ref-filter.c: pass empty-string as NULL to atom parsers