write_file_v(): do not leave incomplete line at the end

All existing callers to this function use it to produce a text file
or an empty file, and a new callsite that mimick them must end their
payload with a LF.  If they forget to do so, the resulting file will
end with an incomplete line.

Teach write_file_v() to complete the incomplete line, if exists, so
that the callers do not have to.

With this, the caller-side fix in builtin/am.c becomes unnecessary.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -199,19 +199,13 @@ static inline const char *am_path(const struct am_state *state, const char *path
 static int write_state_text(const struct am_state *state,
 			    const char *name, const char *string)
 {
-	const char *fmt;
-
-	if (*string && string[strlen(string) - 1] != '\n')
-		fmt = "%s\n";
-	else
-		fmt = "%s";
-	return write_file(am_path(state, name), fmt, string);
+	return write_file(am_path(state, name), "%s", string);
 }
 
 static int write_state_count(const struct am_state *state,
 			     const char *name, int value)
 {
-	return write_file(am_path(state, name), "%d\n", value);
+	return write_file(am_path(state, name), "%d", value);
 }
 
 static int write_state_bool(const struct am_state *state,

wrapper.c

@@ -632,6 +632,7 @@ static int write_file_v(const char *path, int fatal,
 		return -1;
 	}
 	strbuf_vaddf(&sb, fmt, params);
+	strbuf_complete_line(&sb);
 	if (write_in_full(fd, sb.buf, sb.len) != sb.len) {
 		int err = errno;
 		close(fd);