Add -- before %f in the smudge/clean filter configuration

To support filenames starting with dashes.

To update the config of existing repositories, you can re-run git-annex init.

Perhaps it should check every time for the old config and update it, but
that has several problems:

	- read-only repos
	- unexpected commands like `git annex find` changing git configs
	  might be surprising behavior

Since filenames starting with dashes are not super common and the user can
re-init easily enough if their repo needs fixed, I went for the simplest
fix.

doc/bugs/Incorrect_escaping_of_file_names_for_smudging.mdwn

@@ -28,3 +28,5 @@ I'm playing with git-annex on Android and I see the following error messages for
 ### 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)
 
 Sure :)
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/Incorrect_escaping_of_file_names_for_smudging/comment_1_62117387feca4f4b98350a838ed60043._comment

@@ -0,0 +1,26 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2019-03-18T18:03:41Z"
+ content="""
+No need to complicate this with Android, this is sufficient to reproduce
+the problem:
+
+	joey@darkstar:/tmp/a>touch ./-foo
+	joey@darkstar:/tmp/a>git add ./-foo
+	Invalid option `-foo'
+	
+	Usage: git-annex smudge (FILE [--clean] | --update)
+	error: external filter 'git-annex smudge --clean %f' failed 1
+
+The fix is to edit .git/config to contain:
+
+	[filter "annex"]
+	        smudge = git-annex smudge -- %f
+	        clean = git-annex smudge --clean -- %f
+
+The added -- before the filename prevents confusing it with an option.
+
+I've done this for new git-annex repositories, and `git annex init` will
+also update existing repos.
+"""]]