This is actually worse than I thought; when git is being run with a
detached work tree, GIT_INDEX_FILE is treated as a path relative to CWD,
instead of the normal behavior of relative the top of the work tree.
This seems to make it basically impossible for any program that wants to
use GIT_INDEX_FILE to use anything other than an absolute path to it; there
are too many configurations to keep straight that can change how git
interprets what should be a simple relative path to a file.
(I have complained to the git developers.)
This affected git annex view. It turns out that some other places
that use GIT_INDEX_FILE were already working around the bug. I removed the
workaround from Annex.Branch since the new workaround will do.
When git-annex is used with a git version older than 2.2.0, disable support for
adjusted branches, since GIT_COMMON_DIR is needed to update them and was first
added in that version of git.
