Slightly change the multiple patches delete function

The function now first checks whether it can do the operation (i.e. the
applied patches are in consecutive reverse order starting from topmost)
before performing any deletes. It also eliminates the need to check for
local changes with every delete or when the operation is targetted to a
different branch.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>