While manually resolving a merge conflict, being able to run
diff-cache without --cache option between files in the work tree
and either of the ancestor trees is helpful to verify the hand
merged result.  However, diff-cache refuses to handle unmerged
paths, even when run without --cache option.

This changes the behaviour so that the above use case will
report the differences between the compared tree and the magic
0{40} SHA1 (i.e. "look at the work tree").  When there is no
corresponding file in the work tree, or when the command is run
with "--cache" option, it continues to report "unmerged".
Signed-off-by: NJunio C Hamano <junkio@cox.net>