add remote.<name>.annex-checkuuid

Added remote.<name>.annex-checkuuid config, which can be set to false to
disable the default checking of the uuid of remotes that point to
directories. This can be useful to avoid unncessary drive spin-ups and
automounting.

Note that the UUID check is still done before writing to the repository,
to avoid writing to the wrong repository if it got relocated. Check is
also done before checkPresent to avoid getting confused about what is in
which repo. This is effectively the same as the use of git-annex-shell
with a uuid to check that the remote repository is the expected one.
Did not bother with the check for retrieveKeyFile because it doesn't
matter if the wrong repo is used then.

This commit was sponsored by Trenton Cronholm on Patreon.

doc/bugs/Missing_automounts_block_every_command.mdwn

@@ -42,3 +42,6 @@ Use any git-annex command and wait for at least `x-systemd.device-timeout`:
 ### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
 
 I’m very happy with git-annex (thanks) and use it frequently enough to notice this behavior.
+
+> [[fixed|done]] via the `remote.<name>.annex-checkuuid` config setting
+> that can disable this behavior. --[[Joey]]

doc/bugs/Missing_automounts_block_every_command/comment_4_c3cfb1c27806c45dfa6ebf275c175d3e._comment

@@ -0,0 +1,13 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2018-01-10T17:37:42Z"
+ content="""
+Added remote.<name>.annex-checkuuid config, which can be set to false
+to disable the default checking of the uuid. It will still check before
+making any modification of the remote repository.
+
+There may still be situations where using this kind of automount is suboptimal
+with git-annex, as outlined in comment 3, but I think this is as far as it makes
+sense to change git-annex to deal with them.
+"""]]

doc/git-annex.mdwn

@@ -1234,6 +1234,18 @@ Here are all the supported configuration settings.
 
   git-annex caches UUIDs of remote repositories here.
 
+- `remote.<name>.annex-checkuuid`
+
+  This only affects remotes that have their url pointing to a directory on
+  the same system. git-annex normally checks the uuid of such
+  remotes each time it's run, which lets it transparently deal with
+  different drives being mounted to the location at different times.
+
+  Setting annex-checkuuid to false will prevent it from checking the uuid 
+  at startup (although the uuid is still verified before making any
+  changes to the remote repository). This may be useful to set to prevent
+  unncessary spin-up or automounting of a drive.
+
 * `remote.<name>.annex-trustlevel`
 
   Configures a local trust level for the remote. This overrides the value