Print helpful error message when updating depot_tools on Windows

The depot_tools directory may get into a state in which its no longer a
git repo, in which case we can't update it. On Windows this would cause
the update_depot_tools.bat script to fail with a somewhat confusing
error message, this CL fixes that to inform the user what happened and
what they should do about it.

Bug: 1333997
Change-Id: I2c60fc46f1fae7811ff6610e25380d08010d0597
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3718926Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

update_depot_tools.bat

@@ -48,6 +48,13 @@ if errorlevel 1 (
 :: Now clear errorlevel so it can be set by other programs later.
 set errorlevel=
 
+:: Make sure DEPOT_TOOLS_DIR is a git repo
+IF NOT EXIST "%DEPOT_TOOLS_DIR%.git\" (
+  echo Error: Your depot_tools directory does not appear to be a git repository, and cannot be updated.
+  echo Consider deleting your depot_tools directory and following the instructions at https://www.chromium.org/developers/how-tos/install-depot-tools/ to reinstall it.
+  exit /b 1
+)
+
 cd /d "%DEPOT_TOOLS_DIR%."
 call git config remote.origin.fetch > NUL
 for /F %%x in ('git config --get remote.origin.url') DO (