ls-tree: add --full-tree option

ls-tree: add --full-tree option

The established behaviour of "git ls-tree $tree_ish" run from a subdirectory
"sub/dir" in a work tree is to limit the output to the paths in the
subdirectory, and strip off the leading "sub/dir" from the output, since
3c5e846 (ls-tree: major rewrite to do pathspec, 2005-11-26).

This was a "usability" feature made back in the days when the line between
Porcelain and plumbing was blurry, and in retrospect, it probably was
misguided.  The behaviour may be what the end user would expect when the
command is run interactively from a subdirectory, but it also means that a
scripted Porcelain that wants to use the command to list the full contents
of a tree object has to do cd_to_toplevel (and save the output from
"rev-parse --show-prefix" before doing so, so that it can be used as a
pathspec if it wants to limit its operation to the original subdirectory
in other commands).

This new option makes the command operate on the full tree object,
regardless of where in the work tree it is run from.  It also implies the
behaviour that is triggered by the existing --full-name option.

Signed-off-by: Junio C Hamano <gitster@pobox.com>