git-pull.c: introduce git_pull_config()

git-pull.c: introduce git_pull_config()

git-pull makes a seperate call to git_config_get_bool() to read the value
of "rebase.autostash". This can be reduced as a call to git_config() is
already there in the code.

Introduce a callback function git_pull_config() to read "rebase.autostash"
along with other variables.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Paul Tan <pyokagan@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>