merge: Now runs in constant space.

Before, a merge was first calculated, by running various actions that
called git and built up a list of lines, which were at the end sent
to git update-index. This necessarily used space proportional to the size
of the diff between the trees being merged.

Now, lines are streamed into git update-index from each of the actions in
turn.

Runtime size of git-annex merge when merging 50000 location log files
drops from around 100 mb to a constant 4 mb.

Presumably it runs quite a lot faster, too.

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