Fix memory leak when committing millions of changes to the git-annex branch

Eg after git-annex add has run on 2 million files in one go.

Slightly unhappy with the neeed to use a temp file here, but I cannot see
any other alternative (see comments on the bug report).

This commit was sponsored by Hamish Coleman.

doc/bugs/runs_of_of_memory_adding_2_million_files.mdwn

@@ -13,3 +13,5 @@ add 999999 ok
 Stack space overflow: current size 8388608 bytes.
 Use `+RTS -Ksize -RTS' to increase it.
 </pre>
+
+> [[fixed|done]] --[[Joey]]