[win-bootstrap] Make using system Git default

This CL makes searching for an existing system installation of Git
the default behavior.

If a direct Git installation is not found, a warning will be
logged and the bundled Git for Windows will be used instead.

Bug: b/360206460
Change-Id: I48d68b2bf193865ca573199b42e7838c28f1e3cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6068030
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Anne Redulla <aredulla@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

bootstrap/win_tools.bat

@@ -52,7 +52,7 @@ if "%PYTHON3_VERSION%" == "" (
 :: Python is upgraded.
 set BOOTSTRAP_NAME=bootstrap-%PYTHON3_VERSION:.=_%_bin
 set BOOTSTRAP_PATH=%BOOTSTRAP_ROOT_DIR%\%BOOTSTRAP_NAME%
-set BOOTSTRAP_EXTRA_ARGS=--bootstrap-name "%BOOTSTRAP_NAME%"
+set BOOTSTRAP_EXTRA_ARGS=--bootstrap-name "%BOOTSTRAP_NAME%" --use-system-git
 
 :: Install our CIPD packages. The CIPD client self-bootstraps.
 :: See "//cipd.bat" and "//.cipd_impl.ps1" for more information.