git-gui: guard set/unset of GIT_DIR and GIT_WORK_TREE

git-gui unconditionally exports _gitdir as GIT_DIR, and _gitworktree as
GIT_WORK_TREE, to the environment, and unconditionally
unsets these environment variables before invoking gitk or git-gui when
a submodule is involved. This export happens even if _gitworktree is
empty, which happens when running from a bare repository. However,
exporting GIT_WORK_TREE as empty is never valid, and causes errors in
git.

GIT_DIR must be exported if the repository is not discoverable from the
worktree (or current directory if there is no worktree). The user might
have configured this.

If there is a worktree, git-gui makes this the current directory.
However, if the repository sets core.worktree, this value can only be
overridden by GIT_WORK_TREE so the latter must be exported.

As we cannot eliminate conditions where either variable is needed, let's
implement a pair of functions to set / unset these variables without
error, and without ever exporting an empty GIT_WORK_TREE.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

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