Assume a hg repo below a git repo. `git diff` on a file managed by hg, will
return a successful exit value, but no output.
So, if we got a successful exit value from multiple VCS tools, and none of them
gave any output, keep them all as potential candidates. The next time Sy is run,
only these candidates will be tested again.
If one of them returns a proper diff, set b:sy.updated_by to that VCS and prune
all other candidates.
References #235
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