strbuf: split out logic to humanise byte values

strbuf: split out logic to humanise byte values

In a subsequent commit, byte size values displayed in table output for
the git-repo(1) "structure" subcommand will be shown in a more
human-readable format with the appropriate unit prefixes. For this
usecase, the downscaled values and unit strings must be handled
separately to ensure proper column alignment.

Split out logic from strbuf_humanise() to downscale byte values and
determine the corresponding unit prefix into a separate humanise_bytes()
function that provides seperate value and unit strings.

Note that the "byte" string in "t/helper/test-simple-ipc.c" is unmarked
for translation here so that it doesn't conflict with the newly defined
plural "byte/bytes" translation and instead uses it.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>