pull: add pull.autoStash config option

pull: add pull.autoStash config option

Git uses `rebase.autostash` or `merge.autostash` to determine whether a
dirty worktree is allowed during pull. However, this behavior is not
clearly documented, making it difficult for users to discover how to
enable autostash, or causing them to unknowingly enable it. Add new
config option `pull.autostash` along with its documentation and test
cases.

`pull.autostash` provides the same functionality as `rebase.autostash`
and `merge.autostash`, but overrides them when set. If `pull.autostash`
is not set, it falls back to `rebase.autostash` or `merge.autostash`,
depending on the value of `pull.rebase`.

Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>