Fix user expansion in link source

Previous to this patch, having a config like the following would not
work properly:

    - link:
        ~/a: ~/b

This was because the '~' was expanded on the left hand side (the link
target), but not the right hand side (the link source). It was necessary
to use a workaround like this:

    - link:
        ~/a: $HOME/b

This was because variable expansion was being done, but user expansion
was not being done.

This commit adds support for using '~' in the link source.

plugins/link.py

@@ -30,9 +30,10 @@ class Link(dotbot.Plugin):
                 force = source.get('force', force)
                 relink = source.get('relink', relink)
                 create = source.get('create', create)
-                path = os.path.expandvars(source['path'])
+                path = source['path']
             else:
-                path = os.path.expandvars(source)
+                path = source
+            path = os.path.expandvars(os.path.expanduser(path))
             if create:
                 success &= self._create(destination)
             if force or relink:

test/tests/link-environment-user-expansion-target.bash

@@ -0,0 +1,17 @@
+test_description='link expands user in target'
+. '../test-lib.bash'
+
+test_expect_success 'setup' '
+echo "apple" > ~/f
+'
+
+test_expect_success 'run' '
+run_dotbot <<EOF
+- link:
+    ~/g: ~/f
+EOF
+'
+
+test_expect_success 'test' '
+grep "apple" ~/g
+'