ignore: note info/exclude lives in GIT_COMMON_DIR, not GIT_DIR

ignore: note info/exclude lives in GIT_COMMON_DIR, not GIT_DIR

gitignore(5) says that the per-repository ignore file is
$GIT_DIR/info/exclude, but in a worktree that is not the case:

    git rev-parse --git-path info/exclude
    /path/to/main/worktree/.git/info/exclude
    git rev-parse --git-common-dir
    /path/to/main/worktree/.git

We actually use $GIT_COMMON_DIR/info/exclude. Adjust the documentation
and some code comments to say so.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>