run-command: use BUG() to report bugs, not die()

run-command: use BUG() to report bugs, not die()

The slightly misleading name die_bug() of the function intended to
report a bug is actually called always, and only reports a bug if the
passed-in parameter `err` is non-zero.

It uses die_errno() to report the bug, to helpfully include the error
message corresponding to `err`.

However, as these messages indicate bugs, we really should use BUG().
And as BUG() is a macro to be able to report the exact file and line
number, we need to convert die_bug() to a macro instead of only
replacing the die_errno() by a call to BUG().

While at it, use a name more indicative of the purpose: CHECK_BUG().

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