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>