strbuf_addch: avoid calling strbuf_grow

strbuf_addch: avoid calling strbuf_grow

We mark strbuf_addch as inline, because we expect it may be
called from a tight loop. However, the first thing it does
is call the non-inline strbuf_grow(), which can handle
arbitrary-sized growth. Since we know that we only need a
single character, we can use the inline strbuf_avail() to
quickly check whether we need to grow at all.

Our check is redundant when we do call strbuf_grow(), but
that's OK. The common case is that we avoid calling it at
all, and we have made that case faster.

On a silly pathological case:

  perl -le '
    print "[core]";
    print "key$_ = value$_" for (1..1000000)
  ' >input
  git config -f input core.key1

this dropped the time to run git-config from:

  real    0m0.159s
  user    0m0.152s
  sys     0m0.004s

to:

  real    0m0.140s
  user    0m0.136s
  sys     0m0.004s

for a savings of 12%.

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