fix: hide git commit when the build doesn't have git stuff support

This fixes the Git commit string being "GITDIR-NOTFOUND" on the About
page when building a package from the bundled source archive.

Signed-off-by: flow <flowlnlnln@gmail.com>

buildconfig/BuildConfig.cpp.in

@@ -82,6 +82,7 @@ Config::Config()
     {
         GIT_REFSPEC = "refs/heads/stable";
         GIT_TAG = versionString();
+        GIT_COMMIT = "";
     }
 
     if (GIT_REFSPEC.startsWith("refs/heads/"))