luci-py/isolateserver.py: Add support for tar archives when downloading.

appengine/isolate/doc/Design.md

 # Isolated Server
 
 High performance Infrastructure to run an executable with all its dependencies
 mapped in.
 
 **One line description:** How to push 10000 files worth 2Gb in total to tens of
 bots within a few seconds, *even on Windows or Android*.
 
 
 ## Objective
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     will be eventually enforced.
   - `relative_cwd`: relative directory inside the temporary directory tree when
     the command should be executed from.
   - `version`: version of the file format. Increment the minor version for non
     breaking changes and the major version if code written for a previous
     version shouldn't be able to parse it.
 
 
 ##### File types
 
-There are two file types;
+There are the following file types;
   - `basic`: All normal files, the default type.
   - `ar`: An [ar](https://en.wikipedia.org/wiki/Ar_(Unix)) archive containing a
     large number of small files.
+  - `tar`: An [tar](https://en.wikipedia.org/wiki/Tar_(Unix)) archive
+    containing a large number of small files.
 
 
 #### Arbitrary split vs recursive trees
 
 The `.isolated` format supports the `includes` key to split and merge back list
 of files in separate `.isolated` files. It is in stark contrast with more
 traditional tree of trees structure like the git tree object rooted to a git
 commit object.
 
 The reason is to leave a lot of room to the tool generating the `.isolated`
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 ## Testing Plan
 
   - The isolate server code is unit, smoke and canary tested. Since most of the
     isolate server code is OS-independent and written in python, testing is
     relatively easy.
   - Support for hardlinks, symlinks and native path case need OS-specific code
     which can be tested itself on Swarming to get coverage across OSes.
   - A canary Continuous Integration master is run by the chromium team at
     http://build.chromium.org/p/chromium.swarm/waterfall.