update: avoid unnecessarily recreating 'master'

This only affects the behavior of `brew update` while being on a branch
`feature` that doesn't track the upstream branch. For simplicity, the
upstream branch is assumed to be called `master` (`@upstream_branch` in
the code). Consider the following simplified commit history:

        master      origin/master (current state of remote)
        |           |
A---B---C---D---E---F
     \
      G---H---I
              |
              feature (HEAD)

If `origin/master` is equal to `master` and also points at commit `C`,
then `brew update` will update both `master` and `origin/master` to `F`
and report on the changes in the range `C..F`.

However, if `origin/master` is equal to `E` because some commits have
been already fetched with `git fetch origin`, then `brew update` will
recreate `master` from `origin/master` and then pull in the commits from
the remote to update both to `F`. Because `master` gets recreated from
a younger `origin/master`, the report will only contain changes from the
range `E..F` (thus omitting the changes from `C..E`).

This commit adjusts the logic to not recreate `master` if it can be
safely fast-forwarded to `origin/master` (the common case). This fixes
the problem from the second scenario and again reports on the desired
range `C..F`.

Closes Homebrew/homebrew#46951.

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

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