am: shorten ident_split variable name in get_commit_info()

am: shorten ident_split variable name in get_commit_info()

The local ident_split variable is often mentioned three
times per line when dealing with its begin/end pointer
pairs. Let's use a shorter name which lets us get rid of
some long lines.  Since this is a short self-contained
function, readability doesn't suffer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>