update: make stashing local changes more robust
update: make stashing local changes more robust

Users with local changes and without a configured Git identity won't be
able to update Homebrew via `brew update`, as the update will fail when
trying to stash the local modifications with `git stash`. They will be
unable to proceed until they follow Git's advice to configure their
identity or they manage to revert their local changes.

This change always sets a commit e-mail and name, avoiding this issue. A
nice bonus is that experienced Git users can see who created the stash
commit (identifying `brew update` as the author).

Fixes #46930.

Closes #46939.

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

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