apply: simplify build_fake_ancestor()

apply: simplify build_fake_ancestor()

The local variable sha1_ptr in the build_fake_ancestor() function
used to either point at the null_sha1[] (if the ancestor did not
have the path) or at sha1[] (if we read the object name into the
local array), but 7a98869 (apply: get rid of --index-info in favor
of --build-fake-ancestor, 2007-09-17) made the "missing in the
ancestor" case unnecessary, hence sha1_ptr, when used, always points
at the local array.

Get rid of the unneeded variable, and restructure the if/else
cascade a bit to make it easier to read.  There should be no
behaviour change.

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