New strbuf APIs: splice and attach.

New strbuf APIs: splice and attach.

* strbuf_splice replace a portion of the buffer with another.
* strbuf_attach replace a strbuf buffer with the given one, that should be
  malloc'ed. Then it enforces strbuf's invariants. If alloc > len, then this
  function has negligible cost, else it will perform a realloc, possibly
  with a cost.

Also some style issues are fixed now.

Signed-off-by: Pierre Habouzit <redacted>
Acked-by: Linus Torvalds <redacted>
Signed-off-by: Junio C Hamano <redacted>