path: worktree_git_path() should not use file relocation

path: worktree_git_path() should not use file relocation

git_path is a convenience function that usually produces a string
$GIT_DIR/<path>.  Since v2.5.0-rc0~143^2~35 (git_path(): be aware of
file relocation in $GIT_DIR, 2014-11-30), as a side benefit callers
get support for path relocation variables like $GIT_OBJECT_DIRECTORY:

- git_path("index") is $GIT_INDEX_FILE when set
- git_path("info/grafts") is $GIT_GRAFTS_FILE when set
- git_path("objects/<foo>") is $GIT_OBJECT_DIRECTORY/<foo> when set
- git_path("hooks/<foo>") is <foo> under core.hookspath when set
- git_path("refs/<foo>") etc (see path.c::common_list) is relative
  to $GIT_COMMON_DIR instead of $GIT_DIR

worktree_git_path, by comparison, is designed to resolve files in a
specific worktree's git dir.  Unfortunately, it shares code with
git_path and performs the same relocation.  The result is that paths
that are meant to be relative to the specified worktree's git dir end
up replaced by paths from environment variables within the current git
dir.

Luckily, no current callers pass such arguments.  The relocation was
noticed when testing the result of merging two patches under review,
one of which introduces a caller:

* The first patch made git prune check the index file in each
  worktree's git dir (using worktree_git_path(wt, "index")) for
  objects not to prune.  This would trigger the unwanted relocation
  when GIT_INDEX_FILE is set, causing objects reachable from the
  index to be pruned.

* The second patch simplified the relocation logic for index,
  info/grafts, objects, and hooks to happen unconditionally instead of
  based on whether environment or configuration variables are set.
  This caused the relocation to trigger even when GIT_INDEX_FILE is
  not set.

[jn: rewrote commit message; skipping all relocation instead of just
 GIT_INDEX_FILE]

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>