commit: reduce use of redundant global variables

commit: reduce use of redundant global variables

The file-scope global variable head_sha1[] was used to hold the object
name of the current HEAD commit (unless we are about to make an initial
commit). Also there is an independent "static int initial_commit".

Fix all the functions on the call-chain that use these two variables to
take a new "(const) struct commit *current_head" argument instead, and
replace their uses, e.g. "if (initial_commit)" becomes "if (!current_head)"
and a reference to "head_sha1" becomes "current_head->object.sha1".

Signed-off-by: Junio C Hamano <gitster@pobox.com>