Add HOMEBREW_UPDATE_REPORT_ONLY_INSTALLED

This allows reducing the volume of `brew update` output by only
outputting the changes to formulae that you have installed.

Library/Homebrew/cmd/update-report.rb

@@ -456,22 +456,32 @@ class ReporterHub
   private
 
   def dump_formula_report(key, title)
+    only_installed = Homebrew::EnvConfig.update_report_only_installed?
+
     formulae = select_formula(key).sort.map do |name, new_name|
       # Format list items of renamed formulae
       case key
       when :R
         name = pretty_installed(name) if installed?(name)
         new_name = pretty_installed(new_name) if installed?(new_name)
-        "#{name} -> #{new_name}"
+        "#{name} -> #{new_name}" unless only_installed
       when :A
-        name unless installed?(name)
+        name if !installed?(name) && !only_installed
       when :AC
-        name.split("/").last unless cask_installed?(name)
+        name.split("/").last if !cask_installed?(name) && !only_installed
       when :MC, :DC
         name = name.split("/").last
-        cask_installed?(name) ? pretty_installed(name) : name
+        if cask_installed?(name)
+          pretty_installed(name)
+        elsif !only_installed
+          name
+        end
       else
-        installed?(name) ? pretty_installed(name) : name
+        if installed?(name)
+          pretty_installed(name)
+        elsif !only_installed
+          name
+        end
       end
     end.compact
 

Library/Homebrew/env_config.rb

@@ -259,6 +259,10 @@ module Homebrew
         default_text: "macOS: `/private/tmp`, Linux: `/tmp`.",
         default:      HOMEBREW_DEFAULT_TEMP,
       },
+      HOMEBREW_UPDATE_REPORT_ONLY_INSTALLED: {
+        description: "If set, `brew update` only outputs updates to installed software.",
+        boolean:     true,
+      },
       HOMEBREW_UPDATE_TO_TAG:                {
         description: "If set, always use the latest stable tag (even if developer commands " \
                      "have been run).",

docs/Manpage.md

@@ -1703,6 +1703,9 @@ For example, you might add something like the following to your ~/.profile, ~/.b
 
     *Default:* macOS: `/private/tmp`, Linux: `/tmp`.
 
+  * `HOMEBREW_UPDATE_REPORT_ONLY_INSTALLED`:
+    If set, `brew update` only outputs updates to installed software.
+
   * `HOMEBREW_UPDATE_TO_TAG`:
     If set, always use the latest stable tag (even if developer commands have been run).
 

manpages/brew.1

@@ -2345,6 +2345,10 @@ Use this path as the temporary directory for building packages\. Changing this m
 \fIDefault:\fR macOS: \fB/private/tmp\fR, Linux: \fB/tmp\fR\.
 .
 .TP
+\fBHOMEBREW_UPDATE_REPORT_ONLY_INSTALLED\fR
+If set, \fBbrew update\fR only outputs updates to installed software\.
+.
+.TP
 \fBHOMEBREW_UPDATE_TO_TAG\fR
 If set, always use the latest stable tag (even if developer commands have been run)\.
 .