Introduce html.h

All html-functions can be quite easily separated from the rest of cgit, so
lets do it; the only issue was html_filemode which uses some git-defined
macros so the function is moved into ui-shared.c::cgit_print_filemode().

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-diff.c

@@ -7,7 +7,7 @@
  */
 
 #include "cgit.h"
-
+#include "html.h"
 
 unsigned char old_rev_sha1[20];
 unsigned char new_rev_sha1[20];