[RFC] strbuf's in builtin-apply

  I promised it, here it comes. It comes though with some possible
problems, so I'd like to see them discussed first.

  builtin-apply.c was messing with its custom buffers a lot,
reallocating the buffers and so on, hence I had to create strbuf_embed
to have a strbuf-safe API doing exactly the same. Though, because of the
"I want an extra NUL in the end"-invariant, this can come with a quite
high price. in the second patch, there is a hunk:

- nsize = got; - nbuf = convert_to_git(path, buf, &nsize); - if (nbuf) { - free(buf); - *buf_p = nbuf; - *alloc_p = nsize; - *size_p = nsize; - } - return got != size; + + nsize = buf->len; + nbuf = convert_to_git(path, buf->buf, &nsize); + if (nbuf) + strbuf_embed(buf, nbuf, nsize, nsize); + return 0;

  Here, I've not been able to check if convert_to_git was in fact always
dealing with a NUL-terminated buffer (That would be in fact nsize+1) or
not, hence here this strbuf_embed will likely perform a realloc. I don't
know git enough to know if this can become an horrible burden though.

  Another suspicious hunk is:

-	data = (void*) fragment->patch;
[...]
 	case BINARY_LITERAL_DEFLATED:
-		free(desc->buffer);
-		desc->buffer = data;
-		dst_size = fragment->size;
-		break;
+		strbuf_embed(buf, fragment->patch, fragment->size, fragment->size);
+		return 0;

  TTBOMK the ->patch pointer is a pointer inside a buffer, not a buffer
that has been malloc'ed (and there are code paths before my patch that
would still realloc the buffer so I don't think I introduce an issue).
It passes the test-suite without crashing, but well, maybe this should
be a copy instead.

  The rest is pretty straightforward.