git-debrebase: Fix get_tree's memoisation wrt $recurse

git-debrebase: Fix get_tree's memoisation wrt $recurse

$recurse needs to be part of the key to the memo hash!

Luckily, there is only one caller which calls with $recurse==1,
check_series_has_all_patches.  It ends up mishandles this situation by
complaining about the directory names, as if they were unused patches.

And after that no-one needs to call get_tree.

Closes: #907202.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>