The command accesses default_abbrev (defined in environment.c and is
updated via core.abbrev configuration), but never makes any call to
git_config().  The output from "worktree list" ignores the abbrev
setting for this reason.

Make a call to git_config() to read the default set of configuration
variables at the beginning of the command.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>