ensure_bootstrap: always resolve base_dir with dirname

Other depot_tools scripts use dirname regardless of mingw,
so do that here too to simplify.

Bug: 1469696
Change-Id: Ia3c22a4eb72e62564eee1e8b18a88671c8b8b9a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4748852
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Joanna Wang <jojwang@chromium.org>

ensure_bootstrap

@@ -19,11 +19,8 @@
 OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
-if [ $MINGW = 0 ]; then
-  base_dir="${0%/*}"
-else
-  base_dir=$(dirname "${BASH_SOURCE[0]}")
-
+base_dir=$(dirname "${BASH_SOURCE[0]}")
+if [ $MINGW != 0 ]; then
   # Don't bootstrap Python 3 on windows, since it is already done by
   # bootstrap/win_tools.bat.
   if [ "X$DEPOT_TOOLS_BOOTSTRAP_PYTHON3" != "X0" ]; then