strbuf: change an always NULL/"" strbuf_addftime() param to bool

strbuf_addftime() allows callers to pass a time zone name for
expanding %Z. The only current caller either passes the empty string
or NULL, in which case %Z is handed over verbatim to strftime(3).
Replace that string parameter with a flag controlling whether to
remove %Z from the format specification. This simplifies the code.

Commit-message-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

date.c

@@ -243,7 +243,7 @@ const char *show_date(unsigned long time, int tz, const struct date_mode *mode)
 			tm->tm_hour, tm->tm_min, tm->tm_sec, tz);
 	else if (mode->type == DATE_STRFTIME)
 		strbuf_addftime(&timebuf, mode->strftime_fmt, tm, tz,
-				mode->local ? NULL : "");
+				!mode->local);
 	else
 		strbuf_addf(&timebuf, "%.3s %.3s %d %02d:%02d:%02d %d%c%+05d",
 				weekday_names[tm->tm_wday],

strbuf.c

@@ -786,7 +786,7 @@ char *xstrfmt(const char *fmt, ...)
 }
 
 void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
-		     int tz_offset, const char *tz_name)
+		     int tz_offset, int suppress_tz_name)
 {
 	struct strbuf munged_fmt = STRBUF_INIT;
 	size_t hint = 128;
@@ -815,8 +815,7 @@ void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm,
 			fmt++;
 			break;
 		case 'Z':
-			if (tz_name) {
-				strbuf_addstr(&munged_fmt, tz_name);
+			if (suppress_tz_name) {
 				fmt++;
 				break;
 			}

strbuf.h

@@ -343,11 +343,12 @@ extern void strbuf_vaddf(struct strbuf *sb, const char *fmt, va_list ap);
  * `tz_offset` is in decimal hhmm format, e.g. -600 means six hours west
  * of Greenwich, and it's used to expand %z internally.  However, tokens
  * with modifiers (e.g. %Ez) are passed to `strftime`.
- * `tz_name` is used to expand %Z internally unless it's NULL.
+ * `suppress_tz_name`, when set, expands %Z internally to the empty
+ * string rather than passing it to `strftime`.
  */
 extern void strbuf_addftime(struct strbuf *sb, const char *fmt,
 			    const struct tm *tm, int tz_offset,
-			    const char *tz_name);
+			    int suppress_tz_name);
 
 /**
  * Read a given size of data from a FILE* pointer to the buffer.