print slightly better message if git nav-upstream is not called in git repo

Currently, the command will print following message twice when not called
in git repo.

```
fatal: not a git repository (or any parent up to mount point /google)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
```

Bug: 339231299
Change-Id: Iab05e037a81b7e15dcc011d78480cd9ace7c7e36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5544438
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

git-nav-upstream

@@ -3,4 +3,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-git checkout "$(git rev-parse  --abbrev-ref "@{u}")"
+if git rev-parse 2> /dev/null; then
+  git checkout "$(git rev-parse  --abbrev-ref "@{u}")"
+else
+  echo "error: not in a git repository" >&2
+  exit 1
+fi