work around git segfault

Work around bug in git 2.37 that causes a segfault when when
core.untrackedCache is set, and broke git-annex init.

Depending on when git gets fixed and how widely the buggy versions are
used, this could be reverted quite soon, or need to linger for a long time.
It only makes git-annex init a tiny bit slower in a new repo.

Sponsored-by: Max Thoursie on Patreon

Annex/Branch.hs

@@ -133,6 +133,11 @@ getBranch = maybe (hasOrigin >>= go >>= use) return =<< branchsha
 		fromMaybe (error $ "failed to create " ++ fromRef name)
 			<$> branchsha
 	go False = withIndex' True $ do
+		-- Create the index file. This is not necessary,
+		-- except to avoid a bug in git that causes
+		-- git write-tree to segfault when the index file does not
+		-- exist.
+		inRepo $ flip Git.UpdateIndex.streamUpdateIndex []
 		cmode <- annexCommitMode <$> Annex.getGitConfig
 		cmessage <- createMessage
 		inRepo $ Git.Branch.commitAlways cmode cmessage fullname []

CHANGELOG

@@ -11,6 +11,8 @@ git-annex (10.20220725) UNRELEASED; urgency=medium
   * Allow find --branch to be used in a bare repository, the same as
     the deprecated findref can be.
   * add --dry-run: New option. 
+  * Work around bug in git 2.37 that causes a segfault
+    when when core.untrackedCache is set, and broke git-annex init.
 
  -- Joey Hess <id@joeyh.name>  Mon, 25 Jul 2022 15:35:45 -0400
 

doc/bugs/Incompatibility_with_git_2.37/comment_6_3bb6424838d4f8a971f3dce6d1b8dbfb._comment

@@ -0,0 +1,16 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 6"""
+ date="2022-08-04T17:46:32Z"
+ content="""
+Now 2 weeks since the segfault in git has been known, and they have not
+fixed it.
+
+Now feeling that it's might be worth git-annex init avoiding the
+segfault, by making sure to do something that creates the index file before
+write-tree. So I don't have to worry about when this gets fixed in git,
+or the chances that a buggy release will start being widely used for a long
+time.
+
+Yeah, done so.
+"""]]