l10n: bg.po: Updated Bulgarian translation (5230t)

Reduce warnings reported by git-po-helper to 23 which look like false
positives.  Since Bulgarian uses Cyrillic we mark substitutable parts
in capitals rather than quoting with <> which saves two chars per term
(2.5% real estate on a 80 char line) plus it is safer and simpler as
the <> chars have special (sometimes destructive) meaning for the
shells.  It is also easier for the user to parse.  See for example
the message:
  git mailinfo [<options>] <msg> <patch> < mail >info
where both <> characters serve multiple functions.

Signed-off-by: Alexander Shopov <[email protected]>

Author: alshopov