mktree: multiple same-named objects

[David Turner <dturner@twopensource.com>]

git mktree seems to allow the creation of a tree object with multiple
objects of the same name but different SHAs.  This leads to weird
behavior later, unsurprisingly.  For instance, if there are two tree
objects with the same name but different SHAs, the checked out tree will
be the union of them (reasonably), but if you do git add $name, some or
all unmodified files under $name will show up in git status as modified
-- since they differ from one of the parent trees, presumably.

And if different git implementations treat this case differently, then
it might be possible to make a repo that appears to contain one thing
when viewed with one implementation, but contains a different thing for
a different implementation.

Summary: git mktree ought to forbid this, and possibly there ought to be
other checks (for instance, when unpacking) to prevent this.