push-to-pull-request: Use git remote get-url.

This gives us the right behavior when using the `url.*.insteadOf`
mechanism for aliases in Git remote URLs.  For example, if
one's ~/.gitconfig has:

  [url "git@github.com:"]
      insteadOf = gh:

then `git remote add upstream gh:zulip/zulip` will work great, as
the nice, short, mnemonic `gh:` prefix gets expanded to the more
finicky `git@github.com:`.  I use just such a prefix routinely.

But the feature does require that scripts go through the right
abstractions.  In particular `git remote get-url`, since Git 2.7
(from 2016), exists for exactly this reason.  A plain `git config`
command bypasses the expansion, getting the verbatim `gh:...`
version, which doesn't work.

So, switch to that.

As a bonus, we get to behave correctly if for some reason the user
has configured a push URL distinct from the fetch URL for this
remote, just by adding `--push`.  With `git config`, we'd have had
to manually implement the fallback from `remote.upstream.pushUrl` to
`remote.upstream.url` in order to properly handle that case.

tools/push-to-pull-request

@@ -43,7 +43,7 @@ EOF
     exit 1
 fi
 
-remote_url="$(git config remote."$remote".url)"
+remote_url="$(git remote get-url --push "$remote")"
 repo_fq="$(echo "$remote_url" | perl -lne 'print $1 if (
     m, ^ git\@github\.com:
          ([^/]+ / [^/]+?)