solver: hold patches in temporary directory

We can avoid bumping up RLIMIT_NOFILE too much by storing
patches in a temporary directory.  And we can share this
top-level directory with our temporary git repository.

Since we no longer rely on a working-tree for git, we are free
to rearrange the layout and avoid relying on the ".git"
convention and relying on "git -C" for chdir.

This may also ease porting public-inbox to older systems
where git does not support "-C" for chdir.

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