Merge branch 'jk/date-mode-format'

Teach "git log" and friends a new "--date=format:..." option to
format timestamps using system's strftime(3).

* jk/date-mode-format:
  strbuf: make strbuf_addftime more robust
  introduce "format" date-mode
  convert "enum date_mode" into a struct
  show-branch: use DATE_RELATIVE instead of magic number

strbuf.c

@@ -710,3 +710,36 @@ char *xstrfmt(const char *fmt, ...)
 
 	return ret;
 }
+
+void strbuf_addftime(struct strbuf *sb, const char *fmt, const struct tm *tm)
+{
+	size_t hint = 128;
+	size_t len;
+
+	if (!*fmt)
+		return;
+
+	strbuf_grow(sb, hint);
+	len = strftime(sb->buf + sb->len, sb->alloc - sb->len, fmt, tm);
+
+	if (!len) {
+		/*
+		 * strftime reports "0" if it could not fit the result in the buffer.
+		 * Unfortunately, it also reports "0" if the requested time string
+		 * takes 0 bytes. So our strategy is to munge the format so that the
+		 * output contains at least one character, and then drop the extra
+		 * character before returning.
+		 */
+		struct strbuf munged_fmt = STRBUF_INIT;
+		strbuf_addf(&munged_fmt, "%s ", fmt);
+		while (!len) {
+			hint *= 2;
+			strbuf_grow(sb, hint);
+			len = strftime(sb->buf + sb->len, sb->alloc - sb->len,
+				       munged_fmt.buf, tm);
+		}
+		strbuf_release(&munged_fmt);
+		len--; /* drop munged space */
+	}
+	strbuf_setlen(sb, sb->len + len);
+}