remove shadowed variable declaration

An accidental variable declaration ("int enough_space = ..." instead
of "enough_space = " inside a block) shadowing the real one meant we
were always using the copy-to-cache path for patching, even when not
necessary.  Remove it.  Enforce an absolute minimum of free space as
well, to avoid running into problems patching small files, now that
the copy-to-cache path is (inadvertently) well-tested.

Change-Id: Idb7d57241a9adcda2e11001fa44f0cd67ce40d19