brew.sh: make analytics 'curl' fully asynchronous
brew.sh: make analytics 'curl' fully asynchronous

If analytics are enabled and `brew` is used in a command substitution
context, e.g. `brew search` for tab completion, the result is that even
though the Google Analytics `curl` process runs in the background it
still is attached to the captured `stdout`, thus could theoretically
contribute to the result of the command substitution and consequently
makes the command substitution block on the completion of this process.
Redirecting `stdout` (and `stderr` for good measure) to `/dev/null`
makes this process truly asynchronous in these contexts.

Furthermore, even if the process is in the background, it is still
included in the shell's job list and thus shell internals like `wait`
(used in `cmd/update.sh`) have to wait on this process, even if they
never intended to do so. Removing the analytics process from the job
list via `disown` avoids this unintended effect.

Fixes #29.

Closes #34.

Signed-off-by: Martin Afanasjew <54669547a225ff20cba8b75a4adca540eef25858@afanasjew.de>

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