strbuf API additions and enhancements.

Add strbuf_remove, change strbuf_insert:
  As both are special cases of strbuf_splice, implement them as such.
  gcc is able to do the math and generate almost optimal code this way.

Add strbuf_swap:
  Exchange the values of its arguments.
  Use it in fast-import.c

Also fix spacing issues in strbuf.h

Signed-off-by: Pierre Habouzit <madcoder@debian.org>

fast-import.c

@@ -1111,9 +1111,7 @@ static int store_object(
 		if (last->no_swap) {
 			last->data = *dat;
 		} else {
-			struct strbuf tmp = *dat;
-			*dat = last->data;
-			last->data = tmp;
+			strbuf_swap(&last->data, dat);
 		}
 		last->offset = e->offset;
 	}