required: New command, like wanted, but for required content.

Also refactored some code to reduce duplication.

doc/git-annex-preferred-content.mdwn

@@ -10,6 +10,13 @@ using `git annex vicfg` or `git annex wanted`.
 They are used by the `--auto` option, by `git annex sync --content`,
 and by the git-annex assistant.
 
+While preferred content expresses a preference, it can be overridden
+by simply using `git annex drop`. On the other hand, required content
+settings are enforced; `git annex drop` will refuse to drop a file if
+doing so would violate its required content settings. A repository's
+required content can be configured using `git annex vicfg` or
+`git annex required`.
+
 Preferred content expressions are similar, but not identical to
 the [[git-annex-matching-options]](1), just without the dashes.
 For example:

doc/git-annex.mdwn

@@ -264,8 +264,16 @@ subdirectories).
 
 * `groupwanted groupname [expression]`
 
+  Get or set groupwanted expression.
+
   See [[git-annex-groupwanted]](1) for details.
 
+* `required repository [expression]`
+  
+  Get or set required content expression.
+  
+  See [[git-annex-required]](1) for details.
+
 * `schedule repository [expression]`
 
   Get or set scheduled jobs.

doc/required_content.mdwn

@@ -6,7 +6,8 @@ archival repositories, and also require that one copy be stored offsite.
 The format of required content expressions is the same as
 [[preferred_content]] expressions.
 
-Required content settings can be edited using `git annex vicfg`.
+Required content settings can be edited using `git annex vicfg`
+or set using `git annex required`.
 Each repository can have its own settings, and other repositories will
 try to honor those settings when interacting with it.
 

doc/todo/command_line_interface_for_required_content_setthings.mdwn

@@ -9,3 +9,5 @@ used feature, and vicfg can already configure it.
 one when it comes to that. Oh well.)
 
 --[[Joey]]
+
+> [[done]] --[[Joey]]