sideband: allow ANSI SGR with colon-separated subfields

sideband: allow ANSI SGR with colon-separated subfields

The SGR values used for 256-color formatting are officially defined to
be a single field with :-separated subfields (e.g. "\e[1;38:5:XX;40m")
despite the more common but kludgy use of separate values (which then
become context-dependent and lead to misinterpretation by incompatible
terminals).

Signed-off-by: Mantas Mikulėnas <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>