Add an option to git-ls-tree to display also the size of blob

Add an option to git-ls-tree to display also the size of blob

Add -l/--long option to git-ls-tree command, which displays
object size of a blob entry.  Object size is placed after
object id (left-justified with minimum width of 7 characters).
For non-blob entries `-' is used.

Rationale: for non-blob entries size of an object has no much
meaning, and is not very interesting.  Moreover, in planned
pack v4 tree objects would be constructed on demand, so tree
size would need to be calculated.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>