checkout: provide better conflict hunk description with detached HEAD

When running 'git checkout -m' and using diff3 style conflict markers,
we want all the conflict hunks (left-side, "common" or "merge base", and
right-side) to have label markers letting the user know where each came
from.  The "common" hunk label (o.ancestor) came from
old_branch_info->name, but that is NULL when HEAD is detached, which
resulted in a blank label.  Check for that case and provide an
abbreviated commit hash instead.

(Incidentally, this was the only case in the git codebase where
merge_trees() was called with opt->ancestor being NULL.  A subsequent
commit will prevent similar problems by enforcing that merge_trees()
always be called with opt->ancestor != NULL.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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