autoninja: Invoke python3, or batch file on Win.

Bug: 941669
Change-Id: I7f196860d87566cdbd2dda99da151124d9ba917f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2826237
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

autoninja

@@ -4,6 +4,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# In git bash on Windows, invoke the batch file.
+if [ "$(expr substr $(uname -s) 1 10)" == "MINGW64_NT" ]; then
+    autoninja.bat "$@"
+    exit
+fi
+
 # Set unique build ID.
 AUTONINJA_BUILD_ID="$(python3 -c "import uuid; print(uuid.uuid4())")"
 export AUTONINJA_BUILD_ID
@@ -14,8 +20,7 @@ fi
 
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
-# Don't use python3 because it doesn't work in git bash on Windows.
+command=$(python3 "$(dirname -- "$0")/autoninja.py" "$@")
-command=$(python "$(dirname -- "$0")/autoninja.py" "$@")
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
   echo "$command"
 fi