git (post-receive-alternativearch): use unicode for git output

The commit message and diff is stored in `full_change` as a string, via
`read_output()`.  This is passed to `TEXT` which is a unicode string.

When a commit message or diff contains non-ascii characters we get:

    UnicodeDecodeError: 'ascii' codec can't decode byte ...

Encode git output returned from `read_output()` as a unicode string and
define `full_change` as unicode for completeness.

Fixes #6040.

roles/git/hooks/files/post-receive-alternativearch

@@ -66,7 +66,7 @@ def read_output(cmd, abspath, input=None, keepends=False, **kw):
         print(err)
     if not keepends:
         out = out.rstrip('\n\r')
-    return out
+    return out.decode('utf-8')
 
 
 def read_git_output(args, abspath, input=None, keepends=False, **kw):
@@ -164,7 +164,7 @@ def run_as_post_receive_hook():
         if DEBUG:
             print('List of commits:', new_commits_list)
 
-        full_change = ''
+        full_change = u''
         exclude_arch = {}
         for commit in new_commits_list:
             if DEBUG: