Add --strip-trees-with-ids option

[ltrzesniewski]

I was looking for a reliable way to strip a directory based on its path, and this is a quick solution I came up with (since you said in #79 the proper built-in way would be hard to implement).

This simply adds a --strip-trees-with-ids option, based on what --strip-blobs-with-ids does. You may prefer to merge both of these options into a single one, but I didn't want to break existing behavior.

Here's how it's supposed to be used:

git rev-list --all --objects | git cat-file --batch-check='%(objectname) %(objecttype) %(rest)' | grep -Pe '^\w+ tree path/to/the/directory/to/delete$' | cut -d' ' -f1 > ./to-delete.txt
java -jar bfg.jar --no-blob-protection --strip-trees-with-ids ./to-delete.txt

This is much more reliable than the solution in my previous comment here, since it only discards the subtrees, which means if any blob is also used in a different path you don't intent to delete, it won't be affected.

Please note that I absolutely don't know anything about Scala, and just hacked it together without really knowing what I was doing so please bear with me 😄

[jacquesknipper]

Would be very handy to have that option implemented 👍 :)
I'm facing the exact same problem: same blob used for multiple copied files I don't want to remove.
Do you plan to merge this MR?
Thanks!

[jacquesknipper]

I tried with loads of projects and it's working pretty well, except that some of the commits are still referencing non existing files - but it's not a big deal and I'm not sure we can do something about it. It's just noise I would say.