wt-status: avoid strbuf_split*()

wt-status: avoid strbuf_split*()

strbuf is a very good data structure to work with string data
without having to worry about running past the end of the string,
but strbuf_split() is a wrong API and an array of strbuf that the
function produces is a wrong thing to use in general.  You do not
edit these N strings split out of a single strbuf simultaneously.
Often it is much better off to split a string into string_list and
work with the resulting strings.

wt-status.c:abbrev_oid_in_line() takes one line of rebase todo list
(like "pick e813a0200a7121b97fec535f0d0b460b0a33356c title"), and
for instructions that has an object name as the second token on the
line, replace the object name with its unique abbreviation.  After
splitting these tokens out of a single line, no simultaneous edit on
any of these pieces of string that takes advantage of strbuf API
takes place.  The final string is composed with strbuf API, but
these split pieces are merely used as pieces of strings and there is
no need for them to be stored in individual strbuf.

Instead, split the line into a string_list, and compose the final
string using these pieces.

Signed-off-by: Junio C Hamano <gitster@pobox.com>