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>

commit.c

@@ -656,7 +656,7 @@ static char *replace_encoding_header(char *buf, const char *encoding)
 	strbuf_attach(&tmp, buf, strlen(buf), strlen(buf) + 1);
 	if (is_encoding_utf8(encoding)) {
 		/* we have re-coded to UTF-8; drop the header */
-		strbuf_splice(&tmp, start, len, NULL, 0);
+		strbuf_remove(&tmp, start, len);
 	} else {
 		/* just replaces XXXX in 'encoding XXXX\n' */
 		strbuf_splice(&tmp, start + strlen("encoding "),