Fix ocaml#5925

Ensure that if opam has internally synthesised an empty environment
variable, then adding to it doesn't create a stray colon.

src/state/opamEnv.ml

@@ -232,8 +232,20 @@ let apply_op_zip ~sepfmt op arg (rl1,l2 as zip) =
   in
   match op with
   | Eq -> [],[arg]
-  | PlusEq -> [], arg :: rezip zip
-  | EqPlus -> List.rev_append l2 rl1, [arg]
+  | PlusEq ->
+      (* New value goes at head of existing list; no prefix *)
+      begin match rezip zip with
+      | [""] -> [], [arg]
+      | zip -> [], arg::zip
+      end
+  | EqPlus ->
+      (* NB List.rev_append l2 rl1 is equivalent to
+            List.rev (List.rev_append rl1 l2)
+         Place new value at the end *)
+      begin match List.rev_append l2 rl1 with
+      | [""] -> [], [arg]
+      | zip -> zip, [arg]
+      end
   | Cygwin ->
       cygwin (List.rev_append rl1 l2)
   | EqPlusEq -> rl1, arg::l2

tests/reftests/env.test

@@ -192,9 +192,9 @@ Done.
 NV_VARS=
 NV_VARS3=foo:
 NV_VARS4=
-NV_VARS_5925_1=foo:
+NV_VARS_5925_1=foo
 NV_VARS_5925_2=foo
-NV_VARS_5925_3=:foo
+NV_VARS_5925_3=foo
 NV_VARS_5925_4=foo
 NV_VARS_5925_5=foo:
 NV_VARS_5925_6=foo:
@@ -227,9 +227,9 @@ NV_VARS_5926_T_8=a:b
 ### opam env | sort | grep "NV_VARS" | ';' -> ':'
 NV_VARS3='foo:': export NV_VARS3:
 NV_VARS4='': export NV_VARS4:
-NV_VARS_5925_1='foo:': export NV_VARS_5925_1:
+NV_VARS_5925_1='foo': export NV_VARS_5925_1:
 NV_VARS_5925_2='foo': export NV_VARS_5925_2:
-NV_VARS_5925_3=':foo': export NV_VARS_5925_3:
+NV_VARS_5925_3='foo': export NV_VARS_5925_3:
 NV_VARS_5925_4='foo': export NV_VARS_5925_4:
 NV_VARS_5925_5='foo:': export NV_VARS_5925_5:
 NV_VARS_5925_6='foo:': export NV_VARS_5925_6:
@@ -303,9 +303,9 @@ NV_VARS=/another/path
 NV_VARS2=/another/different/path
 NV_VARS3=foo:/yet/another/different/path
 NV_VARS4=
-NV_VARS_5925_1=foo:
+NV_VARS_5925_1=foo
 NV_VARS_5925_2=foo
-NV_VARS_5925_3=:foo
+NV_VARS_5925_3=foo
 NV_VARS_5925_4=foo
 NV_VARS_5925_5=foo:
 NV_VARS_5925_6=foo:
@@ -338,9 +338,9 @@ NV_VARS_5926_T_8=a:b
 ### opam env | sort | grep "NV_VARS" | ';' -> ':'
 NV_VARS3='foo:/yet/another/different/path': export NV_VARS3:
 NV_VARS4='': export NV_VARS4:
-NV_VARS_5925_1='foo:': export NV_VARS_5925_1:
+NV_VARS_5925_1='foo': export NV_VARS_5925_1:
 NV_VARS_5925_2='foo': export NV_VARS_5925_2:
-NV_VARS_5925_3=':foo': export NV_VARS_5925_3:
+NV_VARS_5925_3='foo': export NV_VARS_5925_3:
 NV_VARS_5925_4='foo': export NV_VARS_5925_4:
 NV_VARS_5925_5='foo:': export NV_VARS_5925_5:
 NV_VARS_5925_6='foo:': export NV_VARS_5925_6:
@@ -460,9 +460,9 @@ Done.
 ### opam env --root "$RT" --switch "./$SW" | grep "NV_VARS" | ';' -> ':'
 NV_VARS3='foo:/yet/another/different/path': export NV_VARS3:
 NV_VARS4='': export NV_VARS4:
-NV_VARS_5925_1='foo:': export NV_VARS_5925_1:
+NV_VARS_5925_1='foo': export NV_VARS_5925_1:
 NV_VARS_5925_2='foo': export NV_VARS_5925_2:
-NV_VARS_5925_3=':foo': export NV_VARS_5925_3:
+NV_VARS_5925_3='foo': export NV_VARS_5925_3:
 NV_VARS_5925_4='foo': export NV_VARS_5925_4:
 NV_VARS_5925_5='foo:': export NV_VARS_5925_5:
 NV_VARS_5925_6='foo:': export NV_VARS_5925_6: