Print RUNTIME_PREFIX warning only when GIT_TRACE is set

Print RUNTIME_PREFIX warning only when GIT_TRACE is set

When RUNTIME_PREFIX is enabled, the installation prefix is derived by
trying a limited set of known locations where the git executable can
reside. If none of these is found, a warning is emitted.

When git is built in a directory that matches neither of these known names,
the warning would always be emitted when the uninstalled executable is run.
This is a problem on Windows, where gitk picks the uninstalled git when
invoked from the build directory and gets confused by the warning.

Print the warning only when GIT_TRACE is set.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>