strvec: declare the `strvec_push_nodup()` function globally

strvec: declare the `strvec_push_nodup()` function globally

This function differs from `strvec_push()` in that it takes ownership of
the allocated string that is passed as second argument.

This is useful when appending elements to the string array that have
been freshly allocated and serve no further other purpose after that.

Without declaring this function globally, call sites would allocate the
memory, only to have `strvec_push()` duplicate the string, and then the
first copy would need to be released. Having this function globally
avoids that kind of unnecessary work.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>