strbuf_getwholeline: use getc macro

strbuf_getwholeline calls fgetc in a tight loop. Using the
getc form, which can be implemented as a macro, should be
faster (and we do not care about it evaluating our argument
twice, as we just have a plain variable).

On my glibc system, running "git rev-parse
refs/heads/does-not-exist" on a file with an extremely large
(1.6GB) packed-refs file went from (best of 3 runs):

  real    0m19.383s
  user    0m18.876s
  sys     0m0.528s

to:

  real    0m18.900s
  user    0m18.472s
  sys     0m0.448s

for a wall-clock speedup of 2.5%.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -443,7 +443,7 @@ int strbuf_getwholeline(struct strbuf *sb, FILE *fp, int term)
 		return EOF;
 
 	strbuf_reset(sb);
-	while ((ch = fgetc(fp)) != EOF) {
+	while ((ch = getc(fp)) != EOF) {
 		strbuf_grow(sb, 1);
 		sb->buf[sb->len++] = ch;
 		if (ch == term)