strbuf: add and use strbuf_insertstr()

Add a function for inserting a C string into a strbuf.  Use it
throughout the source to get rid of magic string length constants and
explicit strlen() calls.

Like strbuf_addstr(), implement it as an inline function to avoid the
implicit strlen() calls to cause runtime overhead.

Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -993,7 +993,7 @@ static int do_sign_commit(struct strbuf *buf, const char *keyid)
 			strbuf_insert(buf, inspos, gpg_sig_header, gpg_sig_header_len);
 			inspos += gpg_sig_header_len;
 		}
-		strbuf_insert(buf, inspos++, " ", 1);
+		strbuf_insertstr(buf, inspos++, " ");
 		strbuf_insert(buf, inspos, bol, len);
 		inspos += len;
 		copypos += len;