Grab bag of test_sys_checkout cleanups:

   Doc inside of each test more clearly/consistently.
   TestSysCheckoutSVN didn’t get the inlining-of-helper-methods treatment, now it has that.
   Move various standalone repo helper methods (like create_branch) into a RepoUtils class.
   README.md was missing newlines when rendered as markdown.
   Doc the return value of checkout.main
   Fix test_container_exclude_component - it was looking for the wrong key (which is never present); now it looks for the correct key.

manic/checkout.py

@@ -378,8 +378,8 @@ def main(args):
     the --all option is passed.
 
     Returns a tuple (overall_status, tree_status). overall_status is 0
-    on success, non-zero on failure. tree_status gives the full status
-    if no checkout is happening. If checkout is happening, tree_status
+    on success, non-zero on failure. tree_status is a dict mapping local path
+    to ExternalStatus -- if no checkout is happening. If checkout is happening, tree_status
     is None.
     """
     if args.do_logging:

test/repos/README.md

@@ -7,6 +7,8 @@ git ls-tree --full-tree -r --name-only HEAD
 git cat-file -p HEAD:<filename>
 ```
 
+File contents at a glance:
+```
 container.git/
   readme.txt
 
@@ -28,4 +30,4 @@ mixed-cont-ext.git/
 
 error/
    (no git repo here, just a readme.txt in the clear)
-
+```