strbuf: add and use strbuf_insertstr()

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 <redacted>
Helped-by: Eric Sunshine <redacted>
Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>