There are a number of tests that modify the global or system
search paths during the tests. This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.
There are a few tests that set up a fake home directory and a
fake GLOBAL search path so that we can test things in global
ignore or attribute or config files. This cleans up that code to
work more robustly even if there is a test failure. This also
fixes some valgrind warnings where scanning search paths for
separators could end up doing a little bit of sketchy data access
when coming to the end of search list.
This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.
Ignore rules with slashes in them are matched using FNM_PATHNAME
and use the path to the .gitignore file from the root of the
repository along with the path fragment (including slashes) in
the ignore file itself. Unfortunately, the relative path to the
.gitignore file was being applied to the global core.excludesfile
if that was also named ".gitignore".
This fixes that with more precise matching and includes test for
ignore rules with leading slashes (which were the primary example
of this being broken in the real world).
This also backports an improvement to the file context logic from
the threadsafe-iterators branch where we don't rely on mutating
the key of the attribute file name to generate the context path.
