ls-files: fix bug when recursing with relative pathspec

ls-files: fix bug when recursing with relative pathspec

When using the --recurse-submodules flag with a relative pathspec which
includes "..", an error is produced inside the child process spawned for a
submodule.  When creating the pathspec struct in the child, the ".." is
interpreted to mean "go up a directory" which causes an error stating that the
path ".." is outside of the repository.

While it is true that ".." is outside the scope of the submodule, it is
confusing to a user who originally invoked the command where ".." was indeed
still inside the scope of the superproject.  Since the child process launched
for the submodule has some context that it is operating underneath a
superproject, this error could be avoided.

This patch fixes the bug by passing the 'prefix' to the child process.  Now
each child process that works on a submodule has two points of reference to the
superproject: (1) the 'super_prefix' which is the path from the root of the
superproject down to root of the submodule and (2) the 'prefix' which is the
path from the root of the superproject down to the directory where the user
invoked the git command.

With these two pieces of information a child process can correctly interpret
the pathspecs provided by the user as well as being able to properly format its
output relative to the directory the user invoked the original command from.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>