strbuf: introduce strbuf_getline_{lf,nul}()

The strbuf_getline() interface allows a byte other than LF or NUL as
the line terminator, but this is only because I wrote these
codepaths anticipating that there might be a value other than NUL
and LF that could be useful when I introduced line_termination long
time ago.  No useful caller that uses other value has emerged.

By now, it is clear that the interface is overly broad without a
good reason.  Many codepaths have hardcoded preference to read
either LF terminated or NUL terminated records from their input, and
then call strbuf_getline() with LF or NUL as the third parameter.

This step introduces two thin wrappers around strbuf_getline(),
namely, strbuf_getline_lf() and strbuf_getline_nul(), and
mechanically rewrites these call sites to call either one of
them.  The changes contained in this patch are:

 * introduction of these two functions in strbuf.[ch]

 * mechanical conversion of all callers to strbuf_getline() with
   either '\n' or '\0' as the third parameter to instead call the
   respective thin wrapper.

After this step, output from "git grep 'strbuf_getline('" would
become a lot smaller.  An interim goal of this series is to make
this an empty set, so that we can have strbuf_getline_crlf() take
over the shorter name strbuf_getline().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

credential-cache--daemon.c

@@ -96,12 +96,12 @@ static int read_request(FILE *fh, struct credential *c,
 	static struct strbuf item = STRBUF_INIT;
 	const char *p;
 
-	strbuf_getline(&item, fh, '\n');
+	strbuf_getline_lf(&item, fh);
 	if (!skip_prefix(item.buf, "action=", &p))
 		return error("client sent bogus action line: %s", item.buf);
 	strbuf_addstr(action, p);
 
-	strbuf_getline(&item, fh, '\n');
+	strbuf_getline_lf(&item, fh);
 	if (!skip_prefix(item.buf, "timeout=", &p))
 		return error("client sent bogus timeout line: %s", item.buf);
 	*timeout = atoi(p);