Fix bug importing from a special remote into a subdirectory more than one level deep

Which generated unusual git trees that could confuse git merge,
since they incorrectly had 2 subtrees with the same name.

Root of the bug was a) not testing that at all! but also
b) confusing graftdirs, which contains eg "foo/bar" with
non-recursively read trees, which would contain eg "bar"
when reading a subtree of "foo".

It's worth noting that Annex.Import uses graftTree, but it really
shouldn't have needed to. Eg, when importing into foo/bar from a remote,
it's enough to generate a tree of foo/bar/x, foo/bar/y, and does not
include other files that are at the top of the master branch. It uses
graftTree, so it does include the other files, as well as the foo/bar
tree. git merge will do the same thing for both trees. With that said,
switching it away from graftTree would result in another import
generating a new commit that seems to delete files that were there in a
previous commit, so it probably has to keep using graftTree since it
used it before.

This commit was sponsored by Kevin Mueller on Patreon.

doc/bugs/Importing_into_nested_directory_overwrites_files__63__.mdwn

@@ -100,3 +100,5 @@ supported repository versions: 8
 upgrade supported from repository versions: 0 1 2 3 4 5 6 7
 local repository version: 8
 ```
+
+> [[fixed|done]], thanks for an excellent test case. --[[Joey]]

doc/bugs/Importing_into_nested_directory_overwrites_files__63__/comment_4_c555ea6eeab30eeebb5d33988908118a._comment

@@ -0,0 +1,11 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2021-03-26T18:46:19Z"
+ content="""
+Note that level1/level2 is sufficient for git-annex to generate the bad
+tree. I also saw git merge fail at that depth.
+
+I've fixed this, at least the test case given and some other similar ones
+I thought of to try.
+"""]]