Merge "two fixes for fast-import's 'ls' command" from Jonathan

Merge "two fixes for fast-import's 'ls' command" from Jonathan

Andrew Sayers noticed that the svn-fe | git fast-import pipeline
mishandles a subversion history that copies the root directory to a
sub-directory (e.g. doing `svn cp . trunk` to standardise your
layout).  As David Barr explained, the bug arises when the following
command is sent to git fast-import:

  'ls' SP ':1' SP LF

Instead of reading back what is at the root of r1, it unconditionally
reports the path as missing.

After sleeping on it, here are two patches for 'maint'.  One plugs a
memory leak.  The other ensures that trying to pass an empty path to
the 'ls' command results in an error message that can help the
frontend author instead of the silently broken conversion Andrew
found.

Then we can carefully add 'ls ""' support in 1.7.11.

* commit 'refs/pull-request-tags/jn/maint-fast-import-empty-ls':
  fast-import: don't allow 'ls' of path with empty components
  fast-import: leakfix for 'ls' of dirty trees