trace2: find exec-dir before trace2 initialization

trace2: find exec-dir before trace2 initialization

Teach Git to resolve the executable directory before initializing
Trace2.  This allows the system configuration directory to be
discovered earlier (because it is sometimes relative to the prefix
or runtime-prefix).

This will be used by the next commit to allow trace2 settings to
be loaded from the system config.

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