status: clarify how status.compareBranches deduplicates
The order of output when multiple branches are specified on the
configuration variable was not clearly spelled out in the
documentation.
Add a paragraph to describe the order and also how the branches are
deduplicated. Update t6040 with additional tests to illustrate how
multiple branches are shown and deduplicated.
Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
[jc: made a whole replacement into incremental; wrote log message.] Signed-off-by: Junio C Hamano <gitster@pobox.com>