git fast-export: add --no-data option

git fast-export: add --no-data option

When using git fast-export and git fast-import to rewrite the history
of a repository with large binary files, almost all of the time is
spent dealing with blobs.  This is extremely inefficient if all we want
to do is rewrite the commits and tree structure.  --no-data skips the
output of blobs and writes SHA-1s instead of marks, which provides a
massive speedup.

Signed-off-by: Geoffrey Irving <irving@naml.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>