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>
diff --git a/http.c b/http.c
index 5f34816..00a0e50 100644
--- a/http.c
+++ b/http.c
@@ -680,8 +680,8 @@ static void curl_dump_header(const char *text, unsigned char *ptr, size_t size,
 	for (header = headers; *header; header++) {
 		if (hide_sensitive_header)
 			redact_sensitive_header(*header);
-		strbuf_insert((*header), 0, text, strlen(text));
-		strbuf_insert((*header), strlen(text), ": ", 2);
+		strbuf_insertstr((*header), 0, text);
+		strbuf_insertstr((*header), strlen(text), ": ");
 		strbuf_rtrim((*header));
 		strbuf_addch((*header), '\n');
 		trace_strbuf(&trace_curl, (*header));