git: Fix installing from read-only ${.OBJDIR}

[YONETANI Tomokazu]

On Fri, Aug 28, 2009 at 10:58:40PM -0700, YONETANI Tomokazu wrote:
> commit 8e2e81385b920cc951a0a653361b6a6404a55d5b
>     By default, install command preserves mtime, and files in the source tree

This comment is actually wrong; the install command itself doesn't do it
by default, but tools/install.sh script does, which always adds -p flag to it.
This is the case during buildworld because ${INSTALL} is replaced by
"sh /usr/src/tools/install.sh" via *MAKEENV variables.