Add regression test for ocaml#4861

As in this commit, there should be no output captured from the
opam env --revert invocation.

master_changes.md

@@ -121,6 +121,7 @@ users)
   * env tests: use `sort` to increase stability of the `opam env` output [#5935 @dra27 @rjbou]
   * env.win32: add mixed slashes test [#5935 @dra27]
   * env.win32: add test for environment revert not working correctly for Unix-like variables on Windows [#5935 @dra27]
+  * env.win32: add regression test for reverting additions to PATH-like variables [#5935 @dra27]
 
 ### Engine
   * Add `sort` command [#5935 @dra27]

tests/reftests/env.win32.test

@@ -1,6 +1,31 @@
 N0REP0
 ### : setenv & build env rewriting :
 ### opam switch create rewriting --empty
+### : Test for #4861
+### <pkg:multipath.1>
+opam-version: "2.0"
+setenv: [
+  [ PATH += "XXX:\\" ]
+  [ PATH += "C:\\Devel\\bin1;C:\\Devel\\bin2;C:\\Devel\\bin3;C:\\Devel\\bin4;ZZZ:\\" ]
+]
+x-env-path-rewrite: [
+  [ PATH false ]
+]
+### opam install multipath -yv
+The following actions will be performed:
+=== install 1 package
+  - install multipath 1
+
+<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
+-> installed multipath.1
+Done.
+### : The 'set "P' -> 'set P' rewrite masks differences between whether Path
+### : already contains a directory which needs escaping (e.g. building with
+### : MSVC)
+### opam env --shell=cmd | grep "PATH" | 'XXX:.*' -> '' | 'set "P' -> 'set P'
+set "MANPATH=:"${BASEDIR}/OPAM/rewriting/man""
+set PATH=${BASEDIR}/OPAM/rewriting/bin;C:\Devel\bin1;C:\Devel\bin2;C:\Devel\bin3;C:\Devel\bin4;ZZZ:\;
+### opam exec -- opam env --shell=cmd --revert | grep 'ZZZ:' | 'ZZZ:\\.*' -> 'ZZZ:\' | 'set "P' -> 'set P' 
 ### : Test for #5838
 ### opam env | grep MANPATH
 MANPATH=':"${BASEDIR}/OPAM/rewriting/man"'; export MANPATH;
@@ -603,6 +628,7 @@ x-env-path-rewrite: [
   [ RF_ENVSET_WITH_COL false ]
   [ RF_ENVSET_ADD false ]
   [ RF_ENVSET_ADD_WITH_COL false ]
+  [ PATH false ]
 
   [ RF_ENVBUILD false ]
   [ RF_ENVBUILD_STR false ]