When we use certain Git environment variables to find the Git directory
and the working tree, we want those values to work regardless of what
directory we're in. Currently we don't change the working directory,
but we soon will in order to mimic Git's behavior. In order to make
relative values of these environment variables work correctly once we
change directories, let's turn them into absolute canonical paths when
we set up the repository.
We save the old environment variables into a variable and pass this
variable to the helper for the git lfs env command, which wants to print
the original values, not our modified ones. Note that we can't make the
canonicalization conditional on the subcommand being run because the
configuration structure has been initialized by the time we get to
argument parsing and the configuration code invokes Git upon
initialization.
Because the Cygwin path canonicalization invokes a subprocess on Windows
and the subprocess code caches the environment, we must also reset the
cache after adjusting these environment variables so that future
subprocess invocations, like those for Git, work as expected with the
updated environment.
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