fast-export: do not parse non-commit objects while reading marks file

We read from the marks file and keep only marked commits, but in
order to find the type of object, we are parsing the whole thing,
which is slow, specially in big repositories with lots of big files.

There's no need for that, we can query the object information with
sha1_object_info().

Before this, loading the objects of a fresh emacs import, with 260598
blobs took 14 minutes, after this patch, it takes 3 seconds.

This is the way fast-import does it. Also die if the object is not
found (like fast-import).

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
fast-export: don't parse commits while reading marks file

We don't need the parsed objects at this point, merely the
information that they have marks.

Seems to be three times faster in my setup with lots of objects.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'fc/fast-export-persistent-marks' into pu

Seems to break a handful of topics when merged to the tip of 'pu'.

* fc/fast-export-persistent-marks:
  fast-export: don't parse commits while reading marks file
  fast-export: do not parse non-commit objects while reading marks file