l10n: localizable upload progress messages

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>