Use pretend_sha1_file() in git-blame and git-merge-recursive.

Use pretend_sha1_file() in git-blame and git-merge-recursive.

git-merge-recursive wants an null tree as the fake merge base
while producing the merge result tree.  The null tree does not
have to be written out in the object store as it won't be part
of the result, and it is a prime example for using the new
pretend_sha1_file() function.

git-blame needs to register an arbitrary data to in-core index
while annotating a working tree file (or standard input), but
git-blame is a read-only application and the user of it could
even lack the privilege to write into the object store; it is
another good example for pretend_sha1_file().

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