cogito testsuite failure - git-read-tree too careful

Hello, Petr!

The testsuite for cogito fails in t9300-seek.sh for up-to-date master branches of git and Cogito.

$ ./t9300-seek.sh -v -i
* expecting success: cg-seek cbd273f56aecaaf28b857ae74da77cbb11a4d659
Warning: uncommitted local changes, trying to bring them along
fatal: Entry 'newdir/newfile' not uptodate. Cannot merge.
cg-seek: cbd273f56aecaaf28b857ae74da77cbb11a4d659: bad commit
* FAIL 21: seeking to the first commit
        cg-seek cbd273f56aecaaf28b857ae74da77cbb11a4d659

As I understand it, "git-read-tree -m" in cg-Xlib refuses to merge if there are local changes. This was likely caused by commit fcc387db9bc453dc7e07a262873481af2ee9e5c8:

read-tree -m -u: do not overwrite or remove untracked working tree
files.
    
I guess git-read-tree should be using "--reset" or something to restore
the original behavior.

The "tutorial" testsuite also fails:

Should not be doing an Octopus.
No merge strategy handled the merge.
Merging 5de8995e58b4b478dff476788c3607ed5021fc24 ->
ba8b9edd80500d60d68a6630ee415a3e710f6db2
        to a60f36f73018dc1959d8d2cbd28271f93ee5f686 ...
fatal: Untracked working tree file 'stack.h' would be overwritten by
merge.
cg-merge: git-read-tree failed (merge likely blocked by local changes)
162
?
Unexpected error 4 on line 242

In this case, we may want cg-merge to fail, because it's wrong to overwrite local files without backing them up.