trace: cosmetic fixes for error messages

trace: cosmetic fixes for error messages

The error messages for the trace code are often multi-line;
the first line gets a nice "warning:", but the rest are
left-aligned. Let's give them an indentation to make sure
they stand out as a unit.

While we're here, let's also downcase the first letter of
each error (our usual style), and break up a long line of
advice (since we're already using multiple lines, one more
doesn't hurt).

We also replace "What does 'foo' for GIT_TRACE mean?". While
cute, it's probably a good idea to give more context, and
follow our usual styles. So it's now "unknown trace value
for 'GIT_TRACE': foo".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>