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>

wt-status.c

@@ -988,7 +988,7 @@ static char *read_line_from_git_path(const char *filename)
 		strbuf_release(&buf);
 		return NULL;
 	}
-	strbuf_getline(&buf, fp, '\n');
+	strbuf_getline_lf(&buf, fp);
 	if (!fclose(fp)) {
 		return strbuf_detach(&buf, NULL);
 	} else {
@@ -1076,7 +1076,7 @@ static void read_rebase_todolist(const char *fname, struct string_list *lines)
 	if (!f)
 		die_errno("Could not open file %s for reading",
 			  git_path("%s", fname));
-	while (!strbuf_getline(&line, f, '\n')) {
+	while (!strbuf_getline_lf(&line, f)) {
 		if (line.len && line.buf[0] == comment_line_char)
 			continue;
 		strbuf_trim(&line);