refix bug in a better way

Always run Git.Config.store, so when the git config gets reloaded,
the override gets re-added to it, and changeGitRepo then calls extractGitConfig
on it and sees the annex.* settings from the override.

Remove any prior occurance of -c v and add it to the end. This way,
-c foo=1 -c foo=2 -c foo=1 will pass -c foo=1 to git, rather than -c foo=2

Note that, if git had some multiline config that got built up by
multiple -c's, this would not work still. But it never worked because
before the bug got fixed in the first place, the -c value was repeated
many times, so the multivalue thing would have been wrong. I don't think
-c can be used with multiline configs anyway, though git-config does
talk about them?

Annex.hs

@@ -31,6 +31,7 @@ module Annex (
 	overrideGitConfig,
 	changeGitRepo,
 	adjustGitRepo,
+	addGitConfigOverride,
 	getRemoteGitConfig,
 	withCurrentState,
 	changeDirectory,
@@ -338,6 +339,21 @@ adjustGitRepo a = do
 	changeState $ \s -> s { repoadjustment = \r -> repoadjustment s r >>= a }
 	changeGitRepo =<< gitRepo
 
+{- Adds git config setting, like "foo=bar". It will be passed with -c
+ - to git processes. The config setting is also recorded in the repo,
+ - and the GitConfig is updated. -}
+addGitConfigOverride :: String -> Annex ()
+addGitConfigOverride v = adjustGitRepo $ \r ->
+	Git.Config.store (encodeBS' v) Git.Config.ConfigList $
+		r { Git.gitGlobalOpts = go (Git.gitGlobalOpts r) }
+  where
+	-- Remove any prior occurrance of the setting to avoid
+	-- building up many of them when the adjustment is run repeatedly,
+	-- and add the setting to the end.
+	go [] = [Param "-c", Param v]
+	go (Param "-c": Param v':rest) | v' == v = go rest
+	go (c:rest) = c : go rest
+
 {- Changing the git Repo data also involves re-extracting its GitConfig. -}
 changeGitRepo :: Git.Repo -> Annex ()
 changeGitRepo r = do