Use buffered stdio

Our generation of HTML triggers many small write(2) syscalls
which is inefficient.

Time output on a horrible query against my git.git mirror
shows significant performance improvement:

QUERY_STRING='id=2b93bfac0f5bcabbf60f174f4e7bfa9e318e64d5&id2=d6da71a9d16b8cf27f9d8f90692d3625c849cbc8'
PATH_INFO=/mirrors/git.git/diff
export QUERY_STRING PATH_INFO
time ./cgit >/dev/null

Before:
real    0m1.585s
user    0m0.904s
sys     0m0.658s

After:
real    0m0.750s
user    0m0.666s
sys     0m0.076s

1 files changed, 1 insertions, 1 deletions

diff --git a/html.c b/html.c

index 0bac34b..ced781a 100644
--- a/html.c
+++ b/html.c
@@ -80,7 +80,7 @@ char *fmtalloc(const char *format, ...)
 
 void html_raw(const char *data, size_t size)
 {
-	if (write(STDOUT_FILENO, data, size) != size)
+	if (fwrite(data, 1, size, stdout) != size)
 		die_errno("write error on html output");
 }