Add completions for --fetch-HEAD flag

update.sh: prevent overwriting UPSTREAM_BRANCH.

Closes #656.

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

UPSTREAM_BRANCH was being used both as a loop variable name and name
for the upstream branch for HOMEBREW_REPOSITORY. This meant that the
variable names were overwritten which prevented update.

Closes #693.

Revert "formula_versions: also silence stderr."

formula_versions: also silence stderr.

We already stop old formula versions printing stdout but this also stops
them printing on stderr for e.g. old deprecation warnings.

This would have caught the bug fixed in 985c67.

Otherwise this can prevent taps from being updated as expected.

Update --fetch-HEAD documentation

A failure to change a dylib ID or install name would previously cause a
rather cryptic error message, that didn't include the name of the file
that caused the failure, unless `--debug` was specified. Make sure to
output this information in all cases before re-raising the exception.

This can just live in `brew.sh` and then it doesn’t need repeated in
all the other places.

Tweak the logic further to make the no-op case even faster.

Before:
```
brew update  1.10s user 1.05s system 92% cpu 2.325 total
brew update --preinstall  0.60s user 0.77s system 96% cpu 1.433 total
```

After:
```
brew update  0.60s user 0.34s system 83% cpu 1.132 total
brew update --preinstall  0.29s user 0.24s system 62% cpu 0.860 total
```

These times are now fast enough to avoid any further special-casing for
`--preinstall`, roll it out to users by default and not print a message
unless we've actually found some updates.

Fixes the case where I have `mysql56` installed but do
`brew uninstall foo/bar/mysql56` which isn't a valid formula.

Fixes https://github.com/Homebrew/legacy-homebrew/issues/39883.

Fixes #672.
Fixes #673.

This hasn't been an issue for upstream stable releases for 3 years, since March
2013, and hasn't been an issue for pre-release versions for 4 years.

That release, and indeed the latest modern GPGTools releases, support 10.6 and
above so there's no reason to suspect people are actually encountering the old
versions in the wild with enough regularity to merit this being a permanent part
of our codebase any more.

In the last two years Homebrew has seen one Issue where MacGPG2 was the problem,
and that wasn't reproducible at the time (and still isn't), and the `doctor` check
likely wasn't even raised there. There has only been one Issue where the MacGPG2
`doctor` check was raised in that two year period.

I think it's fair to treat this as an user configuration outlier now rather than
an issue we need to be constantly on guard for.

Ref: Homebrew/legacy-homebrew@dfb171b
Ref: Homebrew/legacy-homebrew#12238
Ref: Homebrew/legacy-homebrew@046498b

Avoid some duplication (thereby also shortening line length) and put
everything in a single string for output via `echo`.

Fixes #671.

This is less than ideal but it gets the time on my machine down from ~6s
to ~2s when checking no taps. It still shows that we're doing way more
in `update.sh` than we need to be doing but that's a future PR.

If a requirement is for a dependent that's already installed and that
dependency is not using a `default_formula` (which would have already
been converted from a `Requirement` to `Dependency` at this stage) then
we want to stop it killing the build.

Read the discussion in https://github.com/Homebrew/homebrew-core/pull/3703

. If you
have a better idea, please file a competing PR. I'm sick to death of discussion.

Closes #662.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes #626.

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