Add flag to make unpack_trees() not print errors.

Add flag to make unpack_trees() not print errors.

(This applies only to errors where a plausible operation is impossible due
to the particular data, not to errors resulting from misuse of the merge
functions.)

This will allow builtin-checkout to suppress merge errors if it's
going to try more merging methods.

Additionally, if unpack_trees() returns with an error, but without
printing anything, it will roll back any changes to the index (by
rereading the index, currently). This obviously could be done by the
caller, but chances are that the caller would forget and debugging
this is difficult. Also, future implementations may give unpack_trees() a
more efficient way of undoing its changes than the caller could.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>