Use Homebrew's gitconfig for various commands

Git is a transport and storage mechanism for us so we don't want to
allow arbitrary customisation of this due to user issues e.g. #3664
and #3561.

Fixes #3664.

Library/Homebrew/cmd/log.rb

@@ -39,6 +39,6 @@ module Homebrew
     end
     args = ARGV.options_only
     args += ["--follow", "--", path] unless path.nil?
-    exec "git", "log", *args
+    Utils.with_homebrew_gitconfig { exec "git", "log", *args }
   end
 end

Library/Homebrew/cmd/update-reset.sh

@@ -28,6 +28,8 @@ EOS
     set -x
   fi
 
+  export HOME="$HOMEBREW_LIBRARY/Homebrew/gitconfig"
+
   for DIR in "$HOMEBREW_REPOSITORY" "$HOMEBREW_LIBRARY"/Taps/*/*
   do
     [[ -d "$DIR/.git" ]] || continue

Library/Homebrew/cmd/update.sh

@@ -19,7 +19,11 @@ git() {
   then
     GIT_EXECUTABLE="$("$HOMEBREW_LIBRARY/Homebrew/shims/scm/git" --homebrew=print-path)"
   fi
+  OLDHOME="$HOME"
+  export HOME="$HOMEBREW_LIBRARY/Homebrew/gitconfig"
   "$GIT_EXECUTABLE" "$@"
+  export HOME="$OLDHOME"
+  unset OLDHOME
 }
 
 git_init_if_necessary() {

Library/Homebrew/download_strategy.rb

@@ -793,15 +793,19 @@ class GitDownloadStrategy < VCSDownloadStrategy
   def update_repo
     return unless @ref_type == :branch || !ref?
 
-    if !shallow_clone? && shallow_dir?
-      quiet_safe_system "git", "fetch", "origin", "--unshallow"
-    else
-      quiet_safe_system "git", "fetch", "origin"
+    Utils.with_homebrew_gitconfig do
+      if !shallow_clone? && shallow_dir?
+        quiet_safe_system "git", "fetch", "origin", "--unshallow"
+      else
+        quiet_safe_system "git", "fetch", "origin"
+      end
     end
   end
 
   def clone_repo
-    safe_system "git", *clone_args
+    Utils.with_homebrew_gitconfig do
+      safe_system "git", *clone_args
+    end
     cached_location.cd do
       safe_system "git", "config", "homebrew.cacheversion", cache_version
       checkout
@@ -811,7 +815,9 @@ class GitDownloadStrategy < VCSDownloadStrategy
 
   def checkout
     ohai "Checking out #{@ref_type} #{@ref}" if @ref_type && @ref
-    quiet_safe_system "git", "checkout", "-f", @ref, "--"
+    Utils.with_homebrew_gitconfig do
+      quiet_safe_system "git", "checkout", "-f", @ref, "--"
+    end
   end
 
   def reset_args
@@ -830,8 +836,10 @@ class GitDownloadStrategy < VCSDownloadStrategy
   end
 
   def update_submodules
-    quiet_safe_system "git", "submodule", "foreach", "--recursive", "git submodule sync"
-    quiet_safe_system "git", "submodule", "update", "--init", "--recursive"
+    Utils.with_homebrew_gitconfig do
+      quiet_safe_system "git", "submodule", "foreach", "--recursive", "git submodule sync"
+      quiet_safe_system "git", "submodule", "update", "--init", "--recursive"
+    end
     fix_absolute_submodule_gitdir_references!
   end
 

Library/Homebrew/gitconfig/.gitconfig

@@ -0,0 +1,14 @@
+[user]
+	# Name used in commit messages.
+	name = Homebrew
+	# Email used in commit messages.
+	email = git@brew.sh
+[fetch]
+	# Always prune when fetching (and pulling).
+	prune = 1
+[core]
+	# Don't prompt for commit messages for merge commits.
+	mergeoptions = --no-edit
+[credential]
+	# Use macOS Keychain to store HTTP passwords.
+	helper = osxkeychain

Library/Homebrew/tap.rb

@@ -230,7 +230,7 @@ class Tap
       ohai "Unshallowing #{name}" unless quiet
       args = %w[fetch --unshallow]
       args << "-q" if quiet
-      path.cd { safe_system "git", *args }
+      Utils.with_homebrew_gitconfig { path.cd { safe_system "git", *args } }
       return
     end
 
@@ -242,7 +242,7 @@ class Tap
     args << "-q" if quiet
 
     begin
-      safe_system "git", *args
+      Utils.with_homebrew_gitconfig { safe_system "git", *args }
       unless Readall.valid_tap?(self, aliases: true)
         unless ARGV.homebrew_developer?
           raise "Cannot tap #{name}: invalid syntax in tap!"

Library/Homebrew/utils/git.rb

@@ -60,6 +60,12 @@ module Utils
     raise "Git is unavailable" unless git_available?
   end
 
+  def self.with_homebrew_gitconfig
+    with_env(HOME: HOMEBREW_LIBRARY/"Homebrew/gitconfig") do
+      yield if block_given?
+    end
+  end
+
   def self.clear_git_available_cache
     @git = nil
     @git_path = nil