Mark messages for translations

Mark messages for translations

Small changes in messages to fit the style and typography of rest.
Reuse already translated messages if possible.
Do not translate messages aimed at developers of git.
Fix unit tests depending on the original string.
Use `test_i18ngrep` for tests with translatable strings.
Change and verify rest of tests via `make GETTEXT_POISON=1 test`.

Signed-off-by: Alexander Shopov <ash@kambanaria.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>