Display prompt for caution only if it is an interactive shell.

Created to fix https://code.google.com/p/skia/issues/detail?id=1197 : Skia Commit Queue quits if the tree is in Caution.

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Review URL: https://codereview.chromium.org/13325002

git-svn-id: http://skia.googlecode.com/svn/trunk@8446 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed