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>

fast-import.c

@@ -1888,7 +1888,7 @@ static int read_next_command(void)
 			struct recent_command *rc;
 
 			strbuf_detach(&command_buf, NULL);
-			stdin_eof = strbuf_getline(&command_buf, stdin, '\n');
+			stdin_eof = strbuf_getline_lf(&command_buf, stdin);
 			if (stdin_eof)
 				return EOF;
 
@@ -1960,7 +1960,7 @@ static int parse_data(struct strbuf *sb, uintmax_t limit, uintmax_t *len_res)
 
 		strbuf_detach(&command_buf, NULL);
 		for (;;) {
-			if (strbuf_getline(&command_buf, stdin, '\n') == EOF)
+			if (strbuf_getline_lf(&command_buf, stdin) == EOF)
 				die("EOF in data (terminator '%s' not found)", term);
 			if (term_len == command_buf.len
 				&& !strcmp(term, command_buf.buf))