Allow empty link sources

If the source for a link is null, use the basename of the destination
with a single leading '.' removed, if present.

README.md

@@ -189,6 +189,49 @@ symbolic link should have a relative path.
       path: zshrc
 ```
 
+If the source location is omitted or set to `null`, Dotbot will use the
+basename of the destination, with a leading `.` stripped if present. This makes
+the following three config files equivalent:
+
+```yaml
+- link:
+    ~/bin/ack: ack
+    ~/.vim: vim
+    ~/.vimrc:
+      relink: true
+      path: vimrc
+    ~/.zshrc:
+      force: true
+      path: zshrc
+```
+
+```yaml
+- link:
+    ~/bin/ack:
+    ~/.vim:
+    ~/.vimrc:
+      relink: true
+    ~/.zshrc:
+      force: true
+```
+
+```json
+[
+  {
+    "link": {
+      "~/bin/ack": null,
+      "~/.vim": null,
+      "~/.vimrc": {
+        "relink": true
+      },
+      "~/.zshrc": {
+        "force": true
+      }
+    }
+  }
+]
+```
+
 ### Shell
 
 Shell commands specify shell commands to be run. Shell commands are run in the

plugins/link.py

@@ -30,9 +30,9 @@ def _process_links(self, links):
                 force = source.get('force', force)
                 relink = source.get('relink', relink)
                 create = source.get('create', create)
-                path = source['path']
+                path = self._default_source(destination, source.get('path'))
             else:
-                path = source
+                path = self._default_source(destination, source)
             path = os.path.expandvars(os.path.expanduser(path))
             if create:
                 success &= self._create(destination)
@@ -46,6 +46,16 @@ def _process_links(self, links):
             self._log.error('Some links were not successfully set up')
         return success
 
+    def _default_source(self, destination, source):
+        if source is None:
+            basename = os.path.basename(destination)
+            if basename.startswith('.'):
+                return basename[1:]
+            else:
+                return basename
+        else:
+            return source
+
     def _is_link(self, path):
         '''
         Returns true if the path is a symbolic link.

test/tests/link-default-source.bash

@@ -0,0 +1,26 @@
+test_description='link uses destination if source is null'
+. '../test-lib.bash'
+
+test_expect_success 'setup' '
+echo "apple" > ${DOTFILES}/f &&
+echo "grape" > ${DOTFILES}/fd
+'
+
+test_expect_success 'run' '
+run_dotbot <<EOF
+- link:
+    ~/f:
+    ~/.f:
+    ~/fd:
+        force: false
+    ~/.fd:
+        force: false
+EOF
+'
+
+test_expect_success 'test' '
+grep "apple" ~/f &&
+grep "apple" ~/.f &&
+grep "grape" ~/fd &&
+grep "grape" ~/.fd
+'