trace: enable tracing by default

Tracing is meant to be extremely low-impact when not enabled.  We
currently ship no tracing calls in libgit2, but if / when we do, the
tracing infrastructure is created to skip tracing as quickly as
possible.  It should compile to a simple test when tracing is off.

Thus, there's on reason to not enable it by default.

CMakeLists.txt

@@ -50,7 +50,7 @@ OPTION(BUILD_EXAMPLES			"Build library usage example apps"			OFF)
 OPTION(BUILD_FUZZERS			"Build the fuzz targets"				OFF)
 OPTION(TAGS				"Generate tags"						OFF)
 OPTION(PROFILE				"Generate profiling information"			OFF)
-OPTION(ENABLE_TRACE			"Enables tracing support"				OFF)
+OPTION(ENABLE_TRACE			"Enables tracing support"				ON)
 OPTION(LIBGIT2_FILENAME			"Name of the produced binary"				OFF)
 OPTION(USE_SSH				"Link with libssh2 to enable SSH support"		 ON)
 OPTION(USE_HTTPS			"Enable HTTPS support. Can be set to a specific backend" ON)