builtin/apply: move 'fn_table' global into 'struct apply_state'

builtin/apply: move 'fn_table' global into 'struct apply_state'

To libify the apply functionality the 'fn_table' variable should
not be static and global to the file. Let's move it into
'struct apply_state'.

As fn_table is cleared at the end of apply_patch(), it is not
necessary to clear it in clear_apply_state().

Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>