fast-import: leakfix for 'ls' of dirty trees

fast-import: leakfix for 'ls' of dirty trees

When the chosen directory has changed since it was last written to
pack, "tree_content_get" makes a deep copy of its content to scribble
on while computing the tree name, which we forgot to free.

This leak has been present since the 'ls' command was introduced in
v1.7.5-rc0~3^2~33 (fast-import: add 'ls' command, 2010-12-02).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>