Re-work ARGV filtering to properly handle --HEAD

Previously, stripping arguments like `--HEAD` for dependencies failed because
that flag affects the installation prefix encoded into formula objects. The
previous implementation of `ARGV` filtering tried to contain all changes to a
single method call before the `FormulaInstaller` forks. This update spreads
things out a bit:

  - The Homebrew `ARGV` extension adds a new method, `filter_for_dependencies`
    which strips flags like `--HEAD`, yields to a block, then restores the
    original contents of ARGV.

  - The `explicitly_requested?` test, which returns true or false depending on
    if a formula object is a member of `ARGV.formulae`, is now a method of
    `Formula` objects.

  - `FormulaInstaller` objects now execute the installation of dependencies
    inside an `ARGV.filter_for_dependencies` block if the dependency was
    `explicitly_requested?`.

Fixes Homebrew/homebrew#8668.
Closes Homebrew/homebrew#7724.

1 files changed, 4 insertions, 0 deletions

diff --git a/Library/Homebrew/cmd/install.rb b/Library/Homebrew/cmd/install.rb
index 7142a5fcb..2848986d4 100644
--- a/Library/Homebrew/cmd/install.rb
+++ b/Library/Homebrew/cmd/install.rb
@@ -84,6 +84,10 @@ module Homebrew extend self
     unless formulae.empty?
       perform_preinstall_checks
       formulae.each do |f|
+        # Check formula status and skip if necessary---a formula passed on the
+        # command line may have been installed to satisfy a dependency.
+        next if f.installed?
+
         begin
           fi = FormulaInstaller.new(f)
           fi.install