Fix freebsd-update(8) erroneous message and exit status when "fetch install" used

[ygy]

Summary

freebsd-update(8) allows chained commands, thus when running "fetch install" and fetch returns 0 with nothing to update, install will fail and the whole command returns 1.

This patch:

• Change the status code to 0 when "freebsd-update install" has nothing to install (aka "Successfully installed nothing")
• Omit the error message "Run '/usr/sbin/freebsd-update fetch' first." of "install" if "fetch" was run in the same command.

This closes PR190660.

[allanjude]

usr.sbin/freebsd-update/freebsd-update.sh

786 ↗

(On Diff #32095)

won't think fork '/bin/true' and use the return value? rather than the literal value "true"?

[ygy]

usr.sbin/freebsd-update/freebsd-update.sh

786 ↗

(On Diff #32095)

I think the no-bracket true here will be attempted to parse as a builtin command or /bin/true but a literal value, but I am not completely sure.

[allanjude]

I think it makes more sense to treat it like $NOTTYOK etc, and just make it 0 or 1

[allanjude]

Hopefully Colin can get a chance to look at this too, but looks good to me now.

[ygy]

@emaste Can I commit it myself?

PR: 190660
Reviewed by: allanjude
Approved by: emaste
Differential_Revision: https://reviews.freebsd.org/D12037

[emaste]

In D12037#260784, @ygy wrote:

@emaste Can I commit it myself?

PR: 190660
Reviewed by: allanjude
Approved by: emaste
Differential_Revision: https://reviews.freebsd.org/D12037

Yes, go ahead (for reference, we've already discussed these non-doc commits with @ygy's mentor).