fast-import: mark strings for translation

Some error or warning messages in "builtin/fast-import.c" are marked
for translation, but many are not.

To be more consistent and provide a better experience to people using a
translated version, let's mark all the remaining error or warning
messages for translation.

While at it, let's make the following small changes:

  - replace "GIT" or "git" in a few error messages to just "Git",
  - replace "Expected from command, got %s" to "expected 'from'
    command, got '%s'", which makes it clearer that "from" is a command
    and should not be translated,
  - downcase error and warning messages that start with an uppercase,
  - fix test cases in "t9300-fast-import.sh" that broke because an
    error or warning message was downcased,
  - split error and warning messages that are too long,
  - adjust the indentation of some arguments of the error functions.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chriscool