fast-import: introduce 'struct fast_import_state'

fast-import: introduce 'struct fast_import_state'

"builtin/fast-import.c" uses a large number of global variables. This
makes it harder than necessary to reason about and improve. Especially
adding new features requires adding more global variables, while
modernizing and eventually libifying the code becomes more and more
difficult.

To start reverting the sad trend to more and more globals and to start
cleaning things up, let's introduce a 'struct fast_import_state' and
pass an instance of it as the first argument to many functions.

This is similar to what was done for "builtin/apply.c" by introducing a
'struct apply_state', see 07d7e290ff (apply: move 'struct apply_state'
to a header file, 2016-08-11) and related commits.

As a first step only the 'global_argc', 'global_argv' and
'global_prefix' variables are moved into the new struct. More variables
will be moved into it in the following commits.

Some functions receive the new 'state' parameter only to pass it
along or for future use, so they are marked with UNUSED for now to
satisfy '-Werror=unused-parameter'.

This is a mostly mechanical refactoring with no intended behavior
change.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>