pull --rebase: add --[no-]autostash flag

pull --rebase: add --[no-]autostash flag

If rebase.autoStash configuration variable is set, there is no way to
override it for "git pull --rebase" from the command line.

Teach "git pull --rebase" the --[no-]autostash command line flag which
overrides the current value of rebase.autoStash, if set. As "git rebase"
understands the --[no-]autostash option, it's just a matter of passing
the option to underlying "git rebase" when "git pull --rebase" is called.

Helped-by: Matthieu Moy <redacted>
Helped-by: Junio C Hamano <redacted>
Helped-by: Paul Tan <redacted>
Helped-by: Eric Sunshine <redacted>
Signed-off-by: Mehul Jain <redacted>
Signed-off-by: Junio C Hamano <redacted>