Fix slash direction: paths in msysgit use /, not \.

BUG=none
TEST=update_depot_tools actually works in msysgit

Review URL: https://codereview.chromium.org/1208053003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295866 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -17,7 +17,7 @@ OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
 if [ $MINGW = 0 ]; then
-  base_dir="${0%\\*}"
+  base_dir="${0%/*}"
 else
   base_dir=$(dirname "$0")
   if [ -L "$base_dir" ]; then