strbuf_getwholeline: avoid calling strbuf_grow

As with the recent speedup to strbuf_addch, we can avoid
calling strbuf_grow() in a tight loop of single-character
adds by instead checking strbuf_avail.

Note that we would instead call strbuf_addch directly here,
but it does more work than necessary: it will NUL-terminate
the result for each character read. Instead, in this loop we
read the characters one by one and then add the terminator
manually at the end.

Running "git rev-parse refs/heads/does-not-exist" on a repo
with an extremely large (1.6GB) packed-refs file went from
(best-of-5):

  real    0m10.948s
  user    0m10.548s
  sys     0m0.412s

to:

  real    0m8.601s
  user    0m8.084s
  sys     0m0.524s

for a wall-clock speedup of 21%.
Helped-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

strbuf.c

@@ -445,6 +445,7 @@ int strbuf_getwholeline(struct strbuf *sb, FILE *fp, int term)
 	strbuf_reset(sb);
 	flockfile(fp);
 	while ((ch = getc_unlocked(fp)) != EOF) {
+		if (!strbuf_avail(sb))
 			strbuf_grow(sb, 1);
 		sb->buf[sb->len++] = ch;
 		if (ch == term)