Fix gitdir detection when in subdir of gitdir

If the current working directory is a subdirectory of the gitdir (e.g.
<repo>/.git/refs/), then setup_git_directory_gently() will climb its
parent directories until it finds itself in a gitdir.  However, no
matter how many parent directories it climbs, it sets
'GIT_DIR_ENVIRONMENT' to ".", which is obviously wrong.

This behaviour affected at least 'git rev-parse --git-dir' and hence
caused some errors in bash completion (e.g. customized command prompt
when on a detached head and completion of refs).

To fix this, we set the absolute path of the found gitdir instead.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -456,7 +456,11 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			inside_git_dir = 1;
 			if (!work_tree_env)
 				inside_work_tree = 0;
-			setenv(GIT_DIR_ENVIRONMENT, ".", 1);
+			if (offset != len) {
+				cwd[offset] = '\0';
+				setenv(GIT_DIR_ENVIRONMENT, cwd, 1);
+			} else
+				setenv(GIT_DIR_ENVIRONMENT, ".", 1);
 			check_repository_format_gently(nongit_ok);
 			return NULL;
 		}

t/t1501-worktree.sh

@@ -92,6 +92,13 @@ cd sub/dir || exit 1
 test_rev_parse 'in repo.git/sub/dir' false true true sub/dir/
 cd ../../../.. || exit 1
 
+test_expect_success 'detecting gitdir when cwd is in a subdir of gitdir' '
+	(expected=$(pwd)/repo.git &&
+	 cd repo.git/refs &&
+	 unset GIT_DIR &&
+	 test "$expected" = "$(git rev-parse --git-dir)")
+'
+
 test_expect_success 'repo finds its work tree' '
 	(cd repo.git &&
 	 : > work/sub/dir/untracked &&