trace: refactor to support multiple env variables

trace: refactor to support multiple env variables

Right now you turn all tracing off and on with GIT_TRACE. To
support new types of tracing without forcing the user to see
all of them, we will soon support turning each tracing area
on with GIT_TRACE_*.

This patch lays the groundwork by providing an interface
which does not assume GIT_TRACE. However, we still maintain
the trace_printf interface so that existing callers do not
need to be refactored.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>