[PATCH] howto/using-merge-subtree: mention --allow-unrelated-histories

Without passing --allow-unrelated-histories the command sequence
fails as intended since commit e379fdf34fee ("merge: refuse to create
too cool a merge by default"). To setup a subtree merging unrelated
histories is normal, so add the option to the howto document.

Signed-off-by: Uwe Kleine-König <redacted>
---
 Documentation/howto/using-merge-subtree.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/howto/using-merge-subtree.txt b/Documentation/howto/using-merge-subtree.txt
index 1ae8d1214ec0..a499a94ac228 100644
--- a/Documentation/howto/using-merge-subtree.txt
+++ b/Documentation/howto/using-merge-subtree.txt

@@ -33,7 +33,7 @@ Here is the command sequence you need:
 
 ----------------
 $ git remote add -f Bproject /path/to/B <1>
-$ git merge -s ours --no-commit Bproject/master <2>
+$ git merge -s ours --no-commit --allow-unrelated-histories Bproject/master <2>
 $ git read-tree --prefix=dir-B/ -u Bproject/master <3>
 $ git commit -m "Merge B project as our subdirectory" <4>
 

-- 
2.19.1