l10n: localizable upload progress messages

Currenly the data rate in throughput_string(...) method is
output by simple strbuf_humanise_bytes(...) call and '/s' append.
But for proper translation of such string the translator needs
full context.

Add strbuf_humanise_rate(...) method to properly print out
localizable version of data rate ('3.5 MiB/s' etc) with full context.

Strings with the units in strbuf_humanise_bytes(...) are marked
for translation.

Signed-off-by: Dimitriy Ryazantcev <dimitriy.ryazantcev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -372,6 +372,12 @@ void strbuf_addbuf_percentquote(struct strbuf *dst, const struct strbuf *src);
  */
 void strbuf_humanise_bytes(struct strbuf *buf, off_t bytes);
 
+/**
+ * Append the given byte rate as a human-readable string (i.e. 12.23 KiB/s,
+ * 3.50 MiB/s).
+ */
+void strbuf_humanise_rate(struct strbuf *buf, off_t bytes);
+
 /**
  * Add a formatted string to the buffer.
  */