fast-import: add (non-)relative-marks feature

fast-import: add (non-)relative-marks feature

After specifying 'feature relative-marks' the paths specified with
'feature import-marks' and 'feature export-marks' are relative to an
internal directory in the current repository.

In git-fast-import this means that the paths are relative to the
'.git/info/fast-import' directory. However, other importers may use a
different location.

Add 'feature non-relative-marks' to disable this behavior, this way
it is possible to, for example, specify the import-marks location as
relative, and the export-marks location as non-relative.

Also add tests to verify this behavior.

Cc: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>