Fixes Homebrew/homebrew#30723.
Fixes Homebrew/homebrew#30724.
Fixes Homebrew/homebrew#30727.

There is always a git executable on PATH due to the git wrapper in
Library/Contributions/cmd. The wrapper will notify the user if there is
not a real git installed.

This option was removed in 8cdf4d8ebf439eb9a9ffcaa0e455ced9459e1e41
because it did not do anything.

Closes Homebrew/homebrew#30678.

When determining whether to remove a symlink during unlinking, we check
three things:

  (a) Is the destination a symlink?
  (b) Does the destination exist?
  (c) Does the destination resolve to the source path?

However, since we know that the source path exists, (b) is guaranteed if
(a) and (c) are true. Thus checking (b) is unnecessary.

Similarly, when creating a new symlink during linking, we first check to
see if the link already exists by checking the same three criteria.
Again, checking (b) is unnecessary here.

See also the expanded test coverage in b52b579b.

Addendum: although we know that the source path exists during unlinking,
it doesn't matter. If the source path does not exist, then we still know
we have a broken symlink pointing into the keg we are unlinking, and
removing that symlink is still safe.