Add git-write-blob.

Add git-write-blob.

A new command, git-write-blob, is introduced.  This registers
the contents of any file on the filesystem as a blob in the
object database and reports its SHA1 to the standard output.
To implement it, the patch promotes index_fd() from a static
function in update-cache.c to extern and moves it to a library
source, sha1_file.c.

This command is used to update git-merge-one-file-script so that
it does not smudge the work tree.

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