Avoid mangling paths given to ExecuteScriptWithArgsAsync

Some extant code was using PowerShell to get the current working
directory and then was hardcoded to prepend that to the given path, in
order to make an "absolute path." This was buggy, so we fixed it by only
prefixing non-rooted paths with the CWD, and do so with `Path.Combine`.

Author: andyleejordan

src/PowerShellEditorServices/Services/PowerShellContext/PowerShellContextService.cs

@@ -1040,26 +1040,28 @@ public async Task ExecuteScriptWithArgsAsync(string script, string arguments = n
 
             if (arguments != null)
             {
-                // Need to determine If the script string is a path to a script file.
-                string scriptAbsPath = string.Empty;
-                try
-                {
-                    // Assume we can only debug scripts from the FileSystem provider
-                    string workingDir = (await ExecuteCommandAsync<PathInfo>(
-                        new PSCommand()
-                            .AddCommand("Microsoft.PowerShell.Management\\Get-Location")
-                            .AddParameter("PSProvider", "FileSystem"),
-                            sendOutputToHost: false,
-                            sendErrorToHost: false).ConfigureAwait(false))
-                        .FirstOrDefault()
-                        .ProviderPath;
-
-                    workingDir = workingDir.TrimEnd(Path.DirectorySeparatorChar);
-                    scriptAbsPath = workingDir + Path.DirectorySeparatorChar + script;
-                }
-                catch (System.Management.Automation.DriveNotFoundException e)
+                // Add CWD from PowerShell if not an absolute path
+                if (!Path.IsPathRooted(script))
                 {
-                    this.logger.LogHandledException("Could not determine current filesystem location", e);
+                    try
+                    {
+                        // Assume we can only debug scripts from the FileSystem provider
+                        string workingDir = (await ExecuteCommandAsync<PathInfo>(
+                            new PSCommand()
+                                .AddCommand("Microsoft.PowerShell.Management\\Get-Location")
+                                .AddParameter("PSProvider", "FileSystem"),
+                                sendOutputToHost: false,
+                                sendErrorToHost: false).ConfigureAwait(false))
+                            .FirstOrDefault()
+                            .ProviderPath;
+
+                        this.logger.LogTrace($"Prepending working directory {workingDir} to script path {script}");
+                        script = Path.Combine(workingDir, script);
+                    }
+                    catch (System.Management.Automation.DriveNotFoundException e)
+                    {
+                        this.logger.LogHandledException("Could not determine current filesystem location", e);
+                    }
                 }
 
                 var strBld = new StringBuilder();
@@ -1071,7 +1073,7 @@ public async Task ExecuteScriptWithArgsAsync(string script, string arguments = n
                 // If the provided path is already quoted, then File.Exists will not find it.
                 // This keeps us from quoting an already quoted path.
                 // Related to issue #123.
-                if (File.Exists(script) || File.Exists(scriptAbsPath))
+                if (File.Exists(script))
                 {
                     // Dot-source the launched script path and single quote the path in case it includes
                     strBld.Append(". ").Append(QuoteEscapeString(script));