* also allow PATH entry to end in / (which is valid)
* in case depot_tools cannot be located in PATH, search for them using which
* added dependency on subprocess (new in python 2.4)

M      git-cl-upload-hook

TEST=run the hook manually with:
1) PATH containing the depot_tools, depot_tools/ (success)
2) checkout of depot_tools into ~/x, and PATH containing that (success)
3) PATH not containing any copy of depot_tools (fail)
4) Several PATH entries containing gclient, not called depot_tools (success)

Review URL: http://codereview.chromium.org/2852032

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@51207 0039d316-1c4b-4281-b951-d872f2087c98