distributed migration for pull and sync --content

pull, sync: When operating on content, automatically hard link objects
that have been migrated.

Added annex.syncmigrations config that can be set to false to prevent
pull and sync from migrating object content.

I think that true is a good default for this config, because it avoids
users having to re-download migrated content or learning about migration.
But, some users will surely not like it, whether because it does take some
time (especially for the first git-annex branch scan when there is a long
history), or because they want to deal with it manually, or because their
filesystem doesn't support hard links and they don't want it to copy
objects.

Sponsored-by: k0ld on Patreon

doc/git-annex-migrate.mdwn

@@ -18,9 +18,10 @@ Note that the content is also still stored using the old keys after
 migration. When possible, hard links are used to avoid that taking up
 extra disk space. Use `git annex unused` to find and remove the old keys.
 
-Normally, nothing will be done to files already using the new backend.
-However, if a backend changes the information it uses to construct a key,
-this can also be used to migrate files to use the new key format.
+Normally, nothing will be done to specified files that are already using
+the new backend. However, if a backend changes the information it uses to
+construct a key, this can also be used to migrate files to use the new key
+format.
 
 # OPTIONS
 
@@ -33,6 +34,9 @@ this can also be used to migrate files to use the new key format.
   This does not modify the working tree, but only hard links
   (or in some cases copies) annex objects to their new keys.
 
+  `git-annex pull` and `git-annex sync --content` automatically do this,
+  unless the `annex.syncmigrations` config is set to false.
+
   Note that older versions of git-annex did not record migrations in a
   way that this can use. Migrations performed with those older versions
   had to be manually run in each clone of the repository.

doc/git-annex-pull.mdwn

@@ -29,7 +29,10 @@ this command will also pull changes from the parent branch.
 When [[git-annex-view]](1) has been used to check out a view branch,
 this command will update the view branch to reflect any changes 
 to the parent branch or metadata.
-  
+
+When [[git-annex-migrate]](1) has been used in other repositories,
+this updates the content in the local repository for those migrations as well.
+
 Normally this tries to download the content of each annexed file,
 from any remote that it's pulling from that has a copy. 
 To control which files it downloads, configure the preferred
@@ -70,12 +73,17 @@ See [[git-annex-preferred-content]](1).
 * `--no-content, `-g`, `--content`
 
   Use `--no-content` or `-g` to avoid downloading (and dropping)
-  the content of annexed files.
+  the content of annexed files, and also prevent doing any migrations of
+  content.
 
   If you often use `--no-content`, you can set the `annex.synccontent`
   configuration to false to prevent downloading content by default.
   The `--content` option overrides that configuration.
 
+  To only prevent only migrations of content, you can set the
+  `annex.syncmigrations` configuration to false.
+  The `--content` option overrides that configuration as well.
+
 * `--content-of=path` `-C path`
 
   Only download (and drop) annexed files in the given path.

doc/git-annex.mdwn

@@ -1222,6 +1222,12 @@ repository, using [[git-annex-config]]. See its man page for a list.)
   To configure the behavior in all clones of the repository,
   this can be set in [[git-annex-config]](1).
 
+* `annex.syncmigrations`
+
+  Set to false to prevent `git-annex sync` and `git-annex pull`
+  from scanning for migrations and updating the local 
+  repository for those migrations.
+
 * `annex.viewunsetdirectory`
 
   This configures the name of a directory that is used in a view to contain