git-write-error with cherry-pick -n usages

Hi,

when you cherry-pick -n a commit that has changes to a file missing in
the current tree, that file will be added as "Unmerged". A subsequent
cherry-pick that tries to pick a commit that has changes to that file
will then fail with:

fatal: git-write-tree: error building trees

I've attached a small shell script to easily reproduce that.

git version 1.5.4.rc2.84.gf85fd

That specific use of cherry-pick might be an user error, but, if
possible, git should give a less cryptic error message.

Björn