strbuf_split_buf(): use ALLOC_GROW()

strbuf_split_buf(): use ALLOC_GROW()

Use ALLOC_GROW() rather than inline code to manage memory in
strbuf_split_buf().  Rename "pos" to "nr" because it better describes
the use of the variable and it better conforms to the "ALLOC_GROW"
idiom.

Also, instead of adding a sentinal NULL value after each entry is
added to the list, only add it once after all of the entries have been
added.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>