bisect: do not run show-branch just to show the current commit

In scripted versions of "git bisect", we used "git show-branch" to
describe a single commit in the bisect log and also to the interactive
user after checking out the next version to be tested.

The former use of "git show-branch" was lost when the helper
function that wrote bisect log entries was rewritten at 0f30233a
(bisect--helper: `bisect_write` shell function in C, 2019-01-02) in
C

But we've kept the latter ever since 0871984d (bisect: make "git
bisect" use new "--next-all" bisect-helper function, 2009-05-09)
started using the faithful C-rewrite introduced at ef24c7ca
(bisect--helper: add "--next-exit" to output bisect results,
2009-04-19).

Showing "[<full hex>] <subject>" is simple enough with our helper
pretty.c::format_commit_message() and spawning show-branch is an
overkill.  Let's lose one external process.

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

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