Simplify the notation for installing by hash to just emsdk install HASH (#767)

Previously this had to be

emsdk install sdk-releases-upstream-HASH

The only thing preventing using just the hash was that there was no
default for the backend, so defaulting to upstream fixes this. And then
we can do

emsdk install HASH

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index bf4986a..22412e2 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -2767,6 +2767,8 @@
           backend = 'fastcomp'
       return 'sdk-releases-%s-%s-64bit' % (backend, release_hash)
     elif len(version) == 40:
+      if backend is None:
+        backend = 'upstream'
       global extra_release_tag
       extra_release_tag = version
       return 'sdk-releases-%s-%s-64bit' % (backend, version)

test/test.py

diff --git a/test/test.py b/test/test.py
index bd21154..8d26669 100755
--- a/test/test.py
+++ b/test/test.py
@@ -259,10 +259,10 @@
 
   def test_install_arbitrary(self):
     # Test that its possible to install arbrary emscripten-releases SDKs
-    run_emsdk('install sdk-releases-upstream-5c776e6a91c0cb8edafca16a652ee1ee48f4f6d2')
+    run_emsdk('install 5c776e6a91c0cb8edafca16a652ee1ee48f4f6d2')
 
     # Check that its not re-downloaded
-    checked_call_with_output(emsdk + ' install sdk-releases-upstream-5c776e6a91c0cb8edafca16a652ee1ee48f4f6d2', expected='Skipped', unexpected='Downloading:')
+    checked_call_with_output(emsdk + ' install 5c776e6a91c0cb8edafca16a652ee1ee48f4f6d2', expected='Skipped', unexpected='Downloading:')
 
 
 if __name__ == '__main__':