Merge branch 'ow/ref-filter-omit-empty'

"git branch --format=..." and "git format-patch --format=..."
learns "--omit-empty" to hide refs that whose formatting result
becomes an empty string from the output.

* ow/ref-filter-omit-empty:
  branch, for-each-ref, tag: add option to omit empty lines

diff --cc Documentation/git-for-each-ref.txt
Simple merge

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/for-each-ref.c
Simple merge

diff --cc builtin/tag.c
Simple merge

diff --cc t/t3203-branch-output.sh
Simple merge

diff --cc t/t6300-for-each-ref.sh
Simple merge

diff --cc t/t7004-tag.sh
Simple merge