optimise adjustTree when adding many TreeItems

optimise adjustTree when adding many TreeItems

The old code traversed the list of addtreeitems once per subdirectory in
the tree, so could get quite slow. Converting to Map lookups sped it up
significantly.

In my test case, git-annex import used to take about 2 minutes, when
calling adjustTree to add back excluded files to the imported tree. This
dropped it down to 6 seconds. Of which 4 seconds are the actual
enumeration of the contents of the remote, so really only 2 seconds for
this.

The path prefix map is a bit suboptimal memory-wise, since items get
stored in the map once per subdirectory on the path to the item. It
would perhaps be better to use a tree data structure.

Also it's suboptimal memory-wise that it builds two maps, as well
as retaining a reference to addtreeitems. I could not see a way around
that though.

Sponsored-by: Luke T. Shumaker on Patreon