Correctly inherit a virtual PWD

PWD is not set in fish vars because it is read only.
Use getenv() to fetch it, allowing fish to inherit a virtual PWD.

Fixes #5525

src/env.cpp

@@ -1005,7 +1005,11 @@ void env_init(const struct config_paths_t *paths /* or NULL */) {
 
     // initialize the PWD variable if necessary
     // Note we may inherit a virtual PWD that doesn't match what getcwd would return; respect that.
-    if (vars.get(L"PWD").missing_or_empty()) {
+    // Note we treat PWD as read-only so it was not set in vars.
+    const char *incoming_pwd = getenv("PWD");
+    if (incoming_pwd && incoming_pwd[0]) {
+        vars.set_one(L"PWD",  ENV_EXPORT | ENV_GLOBAL, str2wcstring(incoming_pwd));
+    } else {
         vars.set_pwd_from_getcwd();
     }
     vars.set_termsize();    // initialize the terminal size variables

tests/cd.err

@@ -4,3 +4,6 @@
 
 ####################
 # cd symlink completion
+
+####################
+# Virtual PWD inheritance

tests/cd.in

@@ -42,3 +42,13 @@ complete -C'cd ../'
 # cd back before removing the test directory again.
 cd $oldpwd
 rm -Rf $base
+
+logmsg Virtual PWD inheritance
+# PWD should be imported and respected by fish
+
+mkdir -p $base/realhome
+set fish_path $PWD/../test/root/bin/fish
+ln -s $base/realhome $base/linkhome
+cd $base/linkhome
+env HOME=$base/linkhome $fish_path -c 'echo PWD is $PWD'
+

tests/cd.out

@@ -17,3 +17,7 @@ cd:
 ../a2/
 ../a3/
 ../rabbithole/
+
+####################
+# Virtual PWD inheritance
+PWD is /tmp/cdcomp_test/linkhome