decorate: refactor format_decorations()

decorate: refactor format_decorations()

Rename the format_decorations_extended function to format_decorations
and drop the format_decorations wrapper macro. Pass the prefix, suffix
and separator strings as a single 'struct format_decorations' pointer
argument instead of separate arguments. Use default values defined in
the function when either the struct pointer or any of the struct fields
are NULL. This is to ease extension with additional options.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>