webapp: support adding existing gcrypt special remotes from removable drives

When adding a removable drive, it's now detected if the drive contains
a gcrypt special remote, and that's all handled nicely. This includes
fetching the git-annex branch from the gcrypt repo in order to find
out how to set up the special remote.

Note that gcrypt repos that are not git-annex special remotes are not
supported. It will attempt to detect such a gcrypt repo and refuse
to use it. (But this is hard to do any may fail; see
https://github.com/blake2-ppc/git-remote-gcrypt/issues/6)

The problem with supporting regular gcrypt repos is that we don't know
what the gcrypt.participants setting is intended to be for the repo.
So even if we can decrypt it, if we push changes to it they might not be
visible to other participants.

Anyway, encrypted sneakernet (or mailnet) is now fully possible with the
git-annex assistant! Assuming that the gpg key distribution is handled
somehow, which the assistant doesn't yet help with.

This commit was sponsored by Navishkar Rao.

Git/Remote.hs

@@ -8,15 +8,21 @@
 module Git.Remote where
 
 import Common
+import Git
+import qualified Git.Command
+import qualified Git.BuildVersion
+
 import Data.Char
 
+type RemoteName = String
+
 {- Construct a legal git remote name out of an arbitrary input string.
  -
  - There seems to be no formal definition of this in the git source,
  - just some ad-hoc checks, and some other things that fail with certian
  - types of names (like ones starting with '-').
  -}
-makeLegalName :: String -> String
+makeLegalName :: String -> RemoteName
 makeLegalName s = case filter legal $ replace "/" "_" s of
 	-- it can't be empty
 	[] -> "unnamed"
@@ -31,3 +37,14 @@ makeLegalName s = case filter legal $ replace "/" "_" s of
 	legal '_' = True
 	legal '.' = True
 	legal c = isAlphaNum c
+	
+remove :: RemoteName -> Repo -> IO ()
+remove remotename = Git.Command.run
+	[ Param "remote"
+	-- name of this subcommand changed
+	, Param $
+		if Git.BuildVersion.older "1.8.0"
+			then "rm"
+			else "remove"
+	, Param remotename
+	]