trace2: create new combined trace facility

Create a new unified tracing facility for git.  The eventual intent is to
replace the current trace_printf* and trace_performance* routines with a
unified set of git_trace2* routines.

In addition to the usual printf-style API, trace2 provides higer-level
event verbs with fixed-fields allowing structured data to be written.
This makes post-processing and analysis easier for external tools.

Trace2 defines 3 output targets.  These are set using the environment
variables "GIT_TR2", "GIT_TR2_PERF", and "GIT_TR2_EVENT".  These may be
set to "1" or to an absolute pathname (just like the current GIT_TRACE).

* GIT_TR2 is intended to be a replacement for GIT_TRACE and logs command
  summary data.

* GIT_TR2_PERF is intended as a replacement for GIT_TRACE_PERFORMANCE.
  It extends the output with columns for the command process, thread,
  repo, absolute and relative elapsed times.  It reports events for
  child process start/stop, thread start/stop, and per-thread function
  nesting.

* GIT_TR2_EVENT is a new structured format. It writes event data as a
  series of JSON records.

Calls to trace2 functions log to any of the 3 output targets enabled
without the need to call different trace_printf* or trace_performance*
routines.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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