update_depot_tools: 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: I9a70975ef5614b1107ac87a5dc29b5ee1a06e298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4750039
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>

update_depot_tools

@@ -25,21 +25,16 @@ if [ $MSYS = 0 ]; then
   echo
 fi
 
+base_dir=$(dirname "$0")
+if [ -e "$base_dir/.disable_auto_update" ]; then
+  exit
+fi
+
 # Test if this script is running under a MinGW install.  If it is, we will
 # hardcode the paths to Git where possible.
 OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
-if [ $MINGW = 0 ]; then
-  base_dir="${0%/*}"
-else
-  base_dir=$(dirname "$0")
-fi
-
-if [ -e "$base_dir/.disable_auto_update" ]; then
-  exit
-fi
-
 # We want to update the bundled tools even under MinGW.
 if [ $MINGW = 0 ]; then
   $COMSPEC //c `cygpath -w "$base_dir/bootstrap/win_tools.bat"`