Workaround for incorrect working directory for Dub

When DMD is used as a Dub dependency and generates the DMD version as
a pre generate command. Dub will execute this command in the current
working directory, which usually is the root package working directory,
instead of the DMD package directory. This will result in the wrong
version, that is, it will pick up the git tag of the root package
instead of the DMD package.

This looks related: https://github.com/dlang/dub/pull/659.

config.d

@@ -50,9 +50,11 @@ string generateVersion(const string versionFile)
 {
     import std.process : execute;
     import std.file : readText;
+    import std.path : dirName;
     import std.string : strip;
 
-    const result = execute(["git", "describe", "--dirty"]);
+    enum workDir = __FILE_FULL_PATH__.dirName;
+    const result = execute(["git", "-C", workDir, "describe", "--dirty"]);
 
     return result.status == 0 ? result.output.strip : versionFile.readText;
 }