[PATCH/RFC] fast-import: add 'ls' command

This patch is by no means complete - I still need to consider the edge cases.
It does achieve the basic requirements for simplifying svn-fe.
The vcs-svn library currently maintains an in-memory index of all paths
in all revisions. Introducing an `ls` command to fast-import allows this
responsibility to be delegated.
Most importantly, it will allow access to the tree data on demand which
is needed for incremental imports.

The two features that svn-fe will need are access the the current in-flight
commit and to previous commits by mark.

 fast-import.c |  127 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 125 insertions(+), 2 deletions(-)