luci-py/isolateserver.py: Add archive support when downloading.

appengine/isolate/doc/Design.md

Index: appengine/isolate/doc/Design.md
diff --git a/appengine/isolate/doc/Design.md b/appengine/isolate/doc/Design.md
index b123cd9b0e973af524fb2b5f00e1863d413cd6dd..1a054bd4494f3c5cbe8797a01cfd141cc45986ad 100644
--- a/appengine/isolate/doc/Design.md
+++ b/appengine/isolate/doc/Design.md
@@ -117,6 +117,7 @@ The root is a dictionary with the following keys:
     - `l`: link destination iff a symlink
     - `m`: posix file mode iff on posix
     - `s`: file size iff not a symlink
+    - `t`: type of the file, defaults to `basic` if not given.

[M-A Ruel, 2016/06/22 17:17:49]

I'd make the statement stronger, it should not be given if the default.

[mithro, 2016/06/23 07:17:21]

Done.

   - `includes`: references another `.isolated` file for additional files or to
     provide the command. In practice, this is used to reduce `.isolated` file
     size by moving rarely changed test data files in a separate `.isolated`
@@ -130,6 +131,14 @@ The root is a dictionary with the following keys:
     version shouldn't be able to parse it.
 
 
+##### File types
+
+There are two file types;
+  - `basic`: All normal files, the default type.
+  - `smallfiles-archive`: An [ar](https://en.wikipedia.org/wiki/Ar_(Unix))

[M-A Ruel, 2016/06/22 17:17:49]

this is overly verbose. :/  I think 'ar' is sufficient.

[mithro, 2016/06/23 07:17:21]

How about "smallar"? If we want to use ar files in the future for different
reason it would be good to have a way to differentiate.

[M-A Ruel, 2016/06/23 13:29:01]

I don't understand, it's an 'ar' file. We use 'ar' files here. If we use ar
files for another reason later, we'll use another value but for now, the file
type is 'ar'.

[mithro, 2016/07/28 16:38:00]

Done.

+    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