env: Translate Windows paths in init shells scripts with cygpath

Co-authored-by: Kate <kit-ty-kate@outlook.com>

master_changes.md

@@ -63,6 +63,7 @@ users)
   * Fix shell detection on Windows when opam is called via Cygwin's /usr/bin/env even though cmd/powershell is used [#5797 @kit-ty-kate]
   * Fix incorrect deduplication of environment variables on update. Effect was that FOO += "" would occlude the value of FOO in the environment [#5837 @dra27]
   * Fix regression from #5356 on the detection of out-of-date environment variables. As part of a refactoring, a filter predicate got inverted [#5837 @dra27]
+  * Unixify Windows paths in init shells scripts (sh, bash, zsh, fish & tsh) [#5797 @rjbou]
 
 ## Opamfile
 

src/state/opamEnv.ml

@@ -875,19 +875,25 @@ let env_hook_script shell =
 
 let source root shell f =
   let fname = OpamFilename.to_string (OpamPath.init root // f) in
+  let unix_transform ?using_backslashes () =
+    let cygpath = Lazy.force OpamSystem.get_cygpath_path_transform in
+    cygpath ~pathlist:false fname
+    |> OpamStd.Env.escape_single_quotes ?using_backslashes
+  in
   match shell with
   | SH_csh ->
-    let fname = OpamStd.Env.escape_single_quotes fname in
-    Printf.sprintf "if ( -f '%s' ) source '%s' >& /dev/null\n" fname fname
+    let fname = unix_transform () in
+    Printf.sprintf "if ( -f '%s' ) source '%s' >& /dev/null\n"
+      fname fname
   | SH_fish ->
-    let fname = OpamStd.Env.escape_single_quotes ~using_backslashes:true fname in
+    let fname = unix_transform ~using_backslashes:true () in
     Printf.sprintf "source '%s' > /dev/null 2> /dev/null; or true\n" fname
   | SH_sh | SH_bash ->
-    let fname = OpamStd.Env.escape_single_quotes fname in
+    let fname = unix_transform () in
     Printf.sprintf "test -r '%s' && . '%s' > /dev/null 2> /dev/null || true\n"
       fname fname
   | SH_zsh ->
-    let fname = OpamStd.Env.escape_single_quotes fname in
+    let fname = unix_transform () in
     Printf.sprintf "[[ ! -r '%s' ]] || source '%s' > /dev/null 2> /dev/null\n"
       fname fname
   | SH_cmd ->