builtin/fmt-merge-msg: stop depending on 'the_repository'

builtin/fmt-merge-msg: stop depending on 'the_repository'

Refactor builtin/fmt-merge-msg.c to remove the dependancy on the global
'the_repository'. Remove the 'UNUSED' macro from the 'struct repository'
parameter and replace 'git_config()' with 'repo_config()' so that
configuration is read from the passed repository. Also, add a test to
make sure that "git fmt-merge-msg -h" can be called outside a
repository.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>