fix reversions due to undocumented and buggy git behavior

* Don't use GIT_PREFIX when GIT_WORK_TREE=. because it seems git
  does not intend GIT_WORK_TREE to be relative to GIT_PREFIX in that
  case, despite GIT_WORK_TREE=.. being relative to GIT_PREFIX.
* Don't use GIT_PREFIX to fix up a relative GIT_DIR, because
  git 2.11 sets GIT_PREFIX set to a path it's not relative to.
  and apparently GIT_DIR is never relative to GIT_PREFIX.

Commit e50ed4ba48 led us down this path
by working around a git bug by relying on the barely documented GIT_PREFIX.

This commit was sponsored by Trenton Cronholm on Patreon.

CHANGELOG

@@ -31,6 +31,12 @@ git-annex (6.20180808) UNRELEASED; urgency=medium
     is configured with exporttree=yes versioning=yes, git-annex can
     download past versions of exported files from it.
   * S3: Support AWS_SESSION_TOKEN.
+  * Don't use GIT_PREFIX when GIT_WORK_TREE=. because it seems git
+    does not intend GIT_WORK_TREE to be relative to GIT_PREFIX in that
+    case, despite GIT_WORK_TREE=.. being relative to GIT_PREFIX.
+  * Don't use GIT_PREFIX to fix up a relative GIT_DIR, because
+    git 2.11 sets GIT_PREFIX set to a path it's not relative to.
+    and apparently GIT_DIR is never relative to GIT_PREFIX.
   * git-annex.cabal: Fix build without assistant, and some other refinements.
     Thanks fftehnik.
 

Git/CurrentRepo.hs

@@ -28,17 +28,17 @@ import Utility.Env.Set
  -
  - Also works around a git bug when running some hooks. It
  - runs the hooks in the top of the repository, but if GIT_WORK_TREE
- - was relative, it then points to the wrong directory. In this situation
- - GIT_PREFIX contains the directory that GIT_WORK_TREE (and GIT_DIR)
- - are relative to.
+ - was relative (but not "."), it then points to the wrong directory.
+ - In this situation GIT_PREFIX contains the directory that 
+ - GIT_WORK_TREE is relative to.
  -}
 get :: IO Repo
 get = do
-	prefix <- getpathenv "GIT_PREFIX"
-	gd <- pathenv "GIT_DIR" prefix
+	gd <- getpathenv "GIT_DIR"
 	r <- configure gd =<< fromCwd
+	prefix <- getpathenv "GIT_PREFIX"
 	wt <- maybe (worktree $ location r) Just
-		<$> pathenv "GIT_WORK_TREE" prefix
+		<$> getpathenvprefix "GIT_WORK_TREE" prefix
 	case wt of
 		Nothing -> return r
 		Just d -> do
@@ -55,10 +55,13 @@ get = do
 				return (Just d)
 			Nothing -> return Nothing
 	
-	pathenv s Nothing = getpathenv s
-	pathenv s (Just prefix) = getpathenv s >>= \case
-		Nothing -> return Nothing
-		Just d -> Just <$> absPath (prefix </> d)
+	getpathenvprefix s (Just prefix) | not (null prefix) =
+		getpathenv s >>= \case
+			Nothing -> return Nothing
+			Just d
+				| d == "." -> return (Just d)
+				| otherwise -> Just <$> absPath (prefix </> d)
+	getpathenvprefix s _ = getpathenv s
 
 	configure Nothing (Just r) = Git.Config.read r
 	configure (Just d) _ = do

doc/bugs/v6_-_under_subdir__58___git_add___34__whines__34____44___git_commit___34__blows__34__.mdwn

@@ -47,3 +47,5 @@ git-annex version: 6.20180807+git230-gaa291acfe-1~ndall+1
 also happens with 6.20180807-1  but not with 6.20170101-1+deb9u2, so it is a regression
 
 [[!meta author=yoh]]
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/v6_-_under_subdir__58___git_add___34__whines__34____44___git_commit___34__blows__34__/comment_5_eaafc970cce2c71ce29f136c90e04504._comment

@@ -0,0 +1,57 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 5"""
+ date="2018-09-11T18:51:07Z"
+ content="""
+Unfortunately git 2.11.0 has the bug that I made git-annex look at
+`GIT_PREFIX` to avoid.
+
+	root@darkstar:/tmp/repo/foo/bar# git --work-tree=../.. ls-files --modified /tmp/repo/
+	BASH_EXECUTION_STRING='set | grep GIT_ >&2; pwd >&2; git-annex smudge --clean '\''subdir/file'\'''
+	GIT_DIR=/tmp/repo/.git
+	GIT_PREFIX=foo/bar/
+	GIT_WORK_TREE=../..
+	/tmp/repo
+
+So, it's not as simple as not looking at `GIT_PREFIX` at all with this
+version of git. When `GIT_WORK_TREE` is relative, and `GIT_PREFIX` is set,
+it needs to combine them to get the actual path to the work tree.
+
+Perhaps git-annex should only use `GIT_PREFIX` 
+for fixup of relative `GIT_WORK_TREE`, but not for `GIT_DIR`. I've so
+far only seen it be needed for `GIT_WORK_TREE`; I only made it also
+be used for `GIT_DIR` out of an assumption git would be consistent in its
+bugginess.
+
+Oh hell, here's another way it fails, with git 2.19:
+
+	joey@darkstar:/tmp/repo/subdir/foo> git --work-tree=../.. status
+	BASH_EXECUTION_STRING='set | grep GIT_ >&2; pwd >&2; git-annex smudge --clean '\''subdir/file'\'''
+	GIT_DIR=/tmp/repo/.git
+	GIT_EXEC_PATH=/usr/lib/git-core
+	GIT_MERGE_AUTOEDIT=no
+	GIT_PAGER=
+	GIT_PREFIX=subdir/foo/
+	GIT_WORK_TREE=.
+	/tmp/repo
+	git-annex: subdir/file: getFileStatus: does not exist (No such file or directory)
+	git-annex: smudge: 1 failed
+
+That one does not involve `GIT_DIR`, instead `GIT_WORK_TREE` is not relative to
+`GIT_PREFIX` so the workaround that assumes it is breaks. I guess that we
+could say that `GIT_DIR` is not relative to some directory in this case,
+so the `GIT_PREFIX` is irrelevant.
+
+Kind of feels like git's behavior is so inconsistent and ill-specified
+that trying to work around the bugs in it is likely to just be a neverending
+source of bugs.
+
+Unfortunately the git devs have so far ignored my bug report despite it
+having an easy test case needing nothing more than a simple shell script to
+reproduce. And there are plenty of differently broken git versions out there
+even if they eventually fix it, so it seems git-annex has to deal with this
+mess somehow.
+
+I'm going with not using `GIT_PREFIX` for `GIT_DIR` and not for
+`GIT_WORK_TREE=.` and hope that's enough.
+"""]]