Promote tests are failing on Travis/OSX

[rgrinberg]

Here's an excerpt:

        diff (internal) (exit 1)
(cd _build/default && /usr/bin/diff -u test/blackbox-tests/test-cases/promote/run.t test/blackbox-tests/test-cases/promote/run.t.corrected)
--- test/blackbox-tests/test-cases/promote/run.t	2018-01-27 14:23:22.000000000 +0000
+++ test/blackbox-tests/test-cases/promote/run.t.corrected	2018-01-27 14:23:33.000000000 +0000
@@ -12,6 +12,9 @@
   toto
 
   $ $JBUILDER build --root . -j1 --diff-command false @blah
+            sh (internal) (exit 1)
+  (cd _build/default && /bin/sh -c 'false x x.gen')
+  [1]
   $ cat x
   toto
 
@@ -23,5 +26,8 @@
   $ cat x
   toto
   $ $JBUILDER build --root . -j1 --diff-command false @blah
+            sh (internal) (exit 1)
+  (cd _build/default && /bin/sh -c 'false x x.gen')
+  [1]
   $ cat x
   toto
+RESULT=1
+'[' 1 -eq 0 ']'
+exit 1

https://travis-ci.org/ocaml/dune/jobs/334103036

cc @diml @dra27 because I can't reproduce on my own Mac.

[ghost]

I found the issue: we are hashing files to detect contents changes. To avoid rehashing files all the times, we check the mtime first. However, on OSX mtime is not precise enough. As a result x wasn't copied back from the source tree to the build directory...