Preserve environment in rebase start command

I ran into a very odd situation where I couldn't rebase because Git
couldn't find my .gitconfig file and wouldn't make any commits. I'm
still not sure exactly what the problem was, but apparently the Git
child process lost some environment variable that was important.
Preserving the environment in the `git rebase` invocation fixes the
problem.

Author: ptomato

tools/rebase-upstream-commits.ts

@@ -167,6 +167,7 @@ try running this tool's "rebasehelp" command.
       // We want this to show up in the same TTY that this tool was launched in.
       stdio: 'inherit',
       env: {
+        ...process.env,
         GIT_SEQUENCE_EDITOR: gitSequenceEditorCmd,
         PATH: process.env['PATH']
       }