PSA: Removing git-tree-prune command from depot_tools

[Andrii Shyshkalov]

If don't know what git-tree-prune is, you can stop reading now.

If you are now curious, look at "git cl archive" or "git rebase-update" instead :)

git-tree-prune is a command in depot_tools hidden inside git_utils, and thus not on the $PATH of most devs.

It was completely untested and as a result got broken. So far, as far as I am aware, only 1 developer noticed this.

Therefore, unless somebody speaks by end of week, I'll kill this code, and recommend to use either of two other tools instead:

* git cl archive

* git map-branches -v -v -v

* git rebase-update (first see colorful tutorial before using it!)

Contact: tandrii@{chromium.org,google.com}

Bug: http://crbug.com/616404 

Best regards,

Andrii

[tan...@google.com]

And it's gone: https://chromium.googlesource.com/chromium/tools/depot_tools.git/+/461a222aac63e606b022ea25a21a755a444d0e4b