The git coming with Xcode 7.0 is too old to know about %P. Most
people will be on newer Xcodes (and Trusty's git and depot_tools's
Windows git are new enough), so this doesn't affect most folks -- so
just complaining loudly is good enough.

BUG=574611
TBR=dpranke

Review-Url: https://codereview.chromium.org/2388483004