travis: unset dev version properly (for upgrade)

Co-authored-by: David Allsopp <david.allsopp@metastack.com>

.travis-ci.sh

@@ -94,8 +94,8 @@ please run make configure and fixup the commit"
 }
 
 unset-dev-version () {
-  # unset git versioning to allow OPAMYES use for upgrade
-  sed -i  -e 's/\(.*with-stdout-to get_git_version.ml.*@@\).*/\1 \\"let version = None\\"")))/' src/client/dune
+  # disable git versioning to allow OPAMYES use for upgrade
+  touch src/client/no-git-version
 }
 
 case "$TARGET" in

src/client/dune

@@ -41,9 +41,14 @@
   (deps    (universe))
   (action  (ignore-stderr (with-stdout-to %{targets} (system "git rev-parse --quiet --verify HEAD || echo .")))))
 
+(rule
+  (targets no-git-version)
+  (mode fallback)
+  (action (copy git-sha %{targets})))
+
 (rule
   ; Travis Upgrade job rely on this line. If changed, update travis-ci.sh too.
-  (with-stdout-to get_git_version.ml (echo "print_string @@ let v = \"%{read-lines:git-sha}\" in if v = \".\" then \"let version = None\" else \"let version = Some \\\"\" ^ v ^ \"\\\"\"")))
+  (with-stdout-to get_git_version.ml (echo "print_string @@ let v = \"%{read-lines:no-git-version}\" in if v = \".\" then \"let version = None\" else \"let version = Some \\\"\" ^ v ^ \"\\\"\"")))
 
 (rule
   (with-stdout-to opamGitVersion.ml (run ocaml %{dep:get_git_version.ml})))