builtin/read-tree: add --recurse-submodules switch

builtin/read-tree: add --recurse-submodules switch

A new known failure mode is introduced[1], which is actually not
a failure but a feature in read-tree. Unlike checkout for which
the recursive submodule tests were originally written, read-tree does
warn about ignored untracked files that would be overwritten.
For the sake of keeping the test library for submodules generic, just
mark the test as a failure.

[1] KNOWN_FAILURE_SUBMODULE_OVERWRITE_IGNORED_UNTRACKED

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>