annex.skipunknown with transition plan

Added annex.skipunknown git config, that can be set to false to change the
behavior of commands like `git annex get foo*`, to not skip over files/dirs
that are not checked into git and are explicitly listed in the command
line.

Significant complexity was needed to handle git-annex add, which uses some
git ls-files calls, but needs to not use --error-unmatch because of course
the files are not known to git.

annex.skipunknown is planned to change to default to false in a
git-annex release in early 2022. There's a todo for that.

Types/GitConfig.hs

@@ -123,6 +123,7 @@ data GitConfig = GitConfig
 	, annexCacheCreds :: Bool
 	, annexAutoUpgradeRepository :: Bool
 	, annexCommitMode :: CommitMode
+	, annexSkipUnknown :: Bool
 	, coreSymlinks :: Bool
 	, coreSharedRepository :: SharedRepository
 	, receiveDenyCurrentBranch :: DenyCurrentBranch
@@ -214,6 +215,7 @@ extractGitConfig configsource r = GitConfig
 	, annexCommitMode = if getbool (annexConfig "allowsign") False
 		then ManualCommit
 		else AutomaticCommit
+	, annexSkipUnknown = getbool (annexConfig "skipunknown") True
 	, coreSymlinks = getbool "core.symlinks" True
 	, coreSharedRepository = getSharedRepository r
 	, receiveDenyCurrentBranch = getDenyCurrentBranch r