branch, for-each-ref, tag: add option to omit empty lines

branch, for-each-ref, tag: add option to omit empty lines

If the given format string expands to the empty string, a newline is
still printed. This makes using the output linewise more tedious. For
example, git update-ref --stdin does not accept empty lines.

Add options to "git branch", "git for-each-ref", and "git tag" to
not print these empty lines.  The default behavior remains the same.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>