subtree: fix "git subtree split --rejoin"

"git merge" in v2.9 prevents merging unrelated histories.

"git subtree split --rejoin" creates unrelated histories when
creating a split repo from a raw sub-directory that did not
originate from an invocation of "git subtree add".

Restore the original behavior by passing --allow-unrelated-histories
when merging subtrees.  This ensures that the synthetic history
created by "git subtree split" can be merged.

Add a test to ensure that this feature works as advertised.

Reported-by: Brett Cundal <brett.cundal@iugome.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions