recovering from "unordered stage entries in index" error

	I've just upgraded my git from 2.0.5 to 2.3.6, and I'm now unable to run 'git svn fetch' in one of my repositories:

$ git svn fetch fatal: unordered stage entries in index write-tree: command returned error: 128

	'git status' shows a few untracked files but is otherwise clean.

	It looks like this check was introduced in 15999d0be8179fb7a2e6eafb931d25ed65df50aa, with the summary "read_index_from(): catch out of order entries when reading an index file" (first appearing in 2.2.0).

	Mailing list discussion looked like it implicated third-party tools.  I don't recall running any other tools on this repo; it doesn't do much day-to-day other than a long series of 'git svn fetch'es.  (But it's been around for a couple of years, so who knows.)

	At any rate, what can I do to recover from this situation?  I tried to locate a path with multiple index entries like this, but got no results:

$ git ls-files -s | cut -f 2-100 | sort | uniq -c | grep -v '^[ \t]*1 '

	(I originally posted on SO at http://stackoverflow.com/questions/30264826/; I'll update that with any solutions that come up here, to ease future googling.)