git-var: constness and globalness cleanup.

git-var: constness and globalness cleanup.

var.c::git_var read function did not have to return writable
strings; make it and the functions it points at return const char *
instead.

ident.c::get_ident() did not need to be global, so make it
static.

Signed-off-by: Junio C Hamano <junkio@cox.net>