Add warning when running gclient under MSYS shell.

update_depot_tools

 #!/usr/bin/env bash
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script will try to sync the bootstrap directories and then defer control.
 
 if [ "$USER" == "root" ];
 then
   echo Running depot tools as root is sad.
   exit
 fi
 
-# Test if this script is running under a MSys install.  If it is, we will
+# Test if this script is running under a MSYS install. This is likely an error
+# if it is, so we warn the user accordingly.
+OUTPUT="$(uname | grep 'MSYS')"
+MSYS=$?
+if [ $MSYS = 0 ]; then
+  echo 'WARNING: It looks like you are running these tools from an MSYS shell'
+  echo '(as opposed to a MinGW shell). This shell is not supported and may'
+  echo 'fail in mysterious ways.'
+  echo
+  echo 'To run the supported MinGW shell, use `git bash`, or use `bin/bash.exe`'
+  echo 'in your MinGW installation, as opposed to `usr/bin/bash.exe`.'
+  echo
+fi
+
+# Test if this script is running under a MinGW install.  If it is, we will
 # hardcode the paths to SVN and Git where possible.
 OUTPUT="$(uname | grep 'MINGW')"
 MINGW=$?
 
 if [ $MINGW = 0 ]; then
   base_dir="${0%/*}"
 else
   base_dir=$(dirname "$0")
   if [ -L "$base_dir" ]; then
     base_dir=`cd "$base_dir" && pwd -P`
   fi
 fi
 
-# Don't try to use Cygwin tools.  Get real win32 tools using the batch script.
-OUTPUT="$(uname | grep 'CYGWIN')"
-CYGWIN=$?
-if [ $CYGWIN = 0 ] || [ $MINGW = 0 ]; then
+# We want to update the bundled tools even under MinGW.
+if [ $MINGW = 0 ]; then
   $COMSPEC /c `cygpath -w "$base_dir/bootstrap/win/win_tools.bat"`
 fi
 
 CANONICAL_GIT_URL="https://chromium.googlesource.com/chromium/tools/depot_tools.git"
 
 SVN="svn"
 if [ -d "$base_dir/svn_bin" -a $MINGW = 0 ]; then
   SVN="$base_dir/svn_bin/svn.exe"
 fi
 
@@ skipping 104 matching lines @@
   "$SVN" -q up "$base_dir"
   AFTER_REVISION=$(get_svn_revision)
   if [[ "$BEFORE_REVISION" != "$AFTER_REVISION" ]]; then
     if [ -z "$DEPOT_TOOLS_HIDE_UPDATED_MESSAGE" ]; then
       echo "Depot Tools has been updated to revision $AFTER_REVISION." 1>&2
     fi
   fi
 fi
 
 find "$base_dir" -iname "*.pyc" -exec rm -f {} \;