fill_tree_descriptor(): mark error message for translation

fill_tree_descriptor(): mark error message for translation

There is an error message in that function to report a missing tree; In
contrast to three other, similar error messages, it is not marked for
translation yet.

Mark it for translation, and while at it, make the error message
consistent with the others by enclosing the SHA in parentheses.

This requires a change to t6030 which expects the previous format of the
commit message. Theoretically, this could present problems with existing
scripts that use `git bisect` and parse its output (because Git does not
provide other means for callers to discern between error conditions).
However, this is unlikely to matter in practice because the most common
course of action to deal with fatal corruptions is to report the error
message to the user and exit, rather than trying to do something with
the reported SHA of the missing tree.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>