gn: Call gn.bat in msysgit on Windows.

Since 'python3' might not exist in Windows, this fixes the 'gn'
script for msysgit users.

Recipe-Manual-Change: infra
Bug: None
Change-Id: Ic5ed76963a7cf1819008470079256a6fd572fb0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3103446Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Jamie Madill <jmadill@chromium.org>

gn

@@ -5,4 +5,10 @@
 
 base_dir=$(dirname "$0")
 
+# In git bash on Windows, invoke the batch file.
+if [ "$(expr "$(uname -s)" : "^MINGW64_NT")" == "10" ]; then
+    gn.bat "$@"
+    exit
+fi
+
 PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/gn.py" "$@"