Merge branch 'master' of ssh://git-annex.branchable.com

doc/bugs/Build_failure__58___Utility__47__QuickCheck.hs__58__38__58__10__58___error__58___Duplicate_instance_declarations.mdwn

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+### Please describe the problem.
+git-annex-6.20170520 no longer builds successfully. My last successful build of git-annex-6.20170520 was Jun 12, 2017, but something has probably changed in at least one of the dependencies on Hackage since then.
+
+The build fails on all three Homebrew CI nodes (macOS 10.10, 10.11, and 10.12).
+
+A full log is here: <https://jenkins.brew.sh/job/Homebrew%20Core%20Pull%20Requests/3175/version=sierra/consoleText>
+
+Duplicate since that will eventually be deleted: <https://gist.github.com/ilovezfs/e93d135243d03567444eb10be8902a95>
+
+### What steps will reproduce the problem?
+Attempt to build git-annex in a cabal sandbox using cabal install.
+
+
+### What version of git-annex are you using? On what operating system?
+git-annex-6.20170520
+
+### Please provide any additional information below.
+
+The build error is
+
+[[!format sh """
+[  8 of 559] Compiling Utility.QuickCheck ( Utility/QuickCheck.hs, dist/dist-sandbox-758c2984/build/git-annex/git-annex-tmp/Utility/QuickCheck.o )
+
+Utility/QuickCheck.hs:38:10: error:
+    Duplicate instance declarations:
+      instance Arbitrary EpochTime
+        -- Defined at Utility/QuickCheck.hs:38:10
+      instance [safe] Arbitrary Foreign.C.Types.CTime
+        -- Defined in ‘Test.QuickCheck.Arbitrary’
+cabal: Leaving directory '.'
+cabal: Error: some packages failed to install:
+git-annex-6.20170520 failed during the building phase. The exception was:
+ExitFailure 1
+"""]]
+
+### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
+
+Yes, thanks!
+