[PATCH/RFC 0/8] refactor trace code

A few weeks ago in:

  http://article.gmane.org/gmane.comp.version-control.git/165496

we discussed the possibility of removing the repo setup trace lines. I
mentioned that I had also recently done some debug code that would not
be appropriate to show all the time with GIT_TRACE. So here is a series
that jumbles it all together: refactors the trace code to handle
multiple environment variables, adds packet-tracing code on its own
variable, and puts the repo setup traces on their own variable.

With this, you can do:

  GIT_TRACE=1 \
  GIT_TRACE_PACKET=/tmp/packet.dump \
  GIT_TRACE_SETUP=0 \
  git whatever

There may be a few other places where it is worth either pushing some
traces to their variable (I think the notes merge code has some
debugging traces in it), or places where existing debugging code could
be enabled (e.g., there is some diffcore debugging code that I sometimes
turn on; it is not even compiled by default, but this would give a nice
way of pushing that decision to runtime).

I have mixed feelings on adding a bunch of debugging code. On the one
hand, it's mostly dead code that nobody runs. On the other hand, it is
sometimes really helpful to be able to tell a user "run with this
environment variable and tell us what it says" without having to get
them to apply a custom patch.

  [1/8]: compat: provide a fallback va_copy definition
  [2/8]: strbuf: add strbuf_addv
  [3/8]: trace: add trace_vprintf
  [4/8]: trace: refactor to support multiple env variables
  [5/8]: trace: factor out "do we want to trace" logic
  [6/8]: trace: add trace_strbuf
  [7/8]: add packet tracing debug code
  [8/8]: trace: give repo_setup trace its own key

-Peff