Fix #52: Add ability to checkout a branch or tag.

GitDownloadStrategy and MercurialDownloadStrategy
now can be used like this:

  head 'git://server/repo.git', :branch => 'stable'
  head 'hg://server/repo/', :tag => '1.0.4'

2 files changed, 37 insertions, 6 deletions

diff --git a/Library/Homebrew/download_strategy.rb b/Library/Homebrew/download_strategy.rb
index 3b9a9dd49..fc5cdfef0 100644
--- a/Library/Homebrew/download_strategy.rb
+++ b/Library/Homebrew/download_strategy.rb
@@ -22,8 +22,15 @@
 #  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #
 class AbstractDownloadStrategy
-  def initialize url, name, version
+  def initialize url, name, version, specs
     @url=url
+    case specs
+    when Hash
+      @spec = specs.keys.first # only use first spec
+      @ref = specs.values.first
+    else
+      spec = nil
+    end
     @unique_token="#{name}-#{version}" unless name.to_s.empty? or name == '__UNKNOWN__'
   end
 end
@@ -131,8 +138,17 @@ class GitDownloadStrategy <AbstractDownloadStrategy
     end
   end
   def stage
-    dst=Dir.getwd
+    dst = Dir.getwd
     Dir.chdir @clone do
+      if @spec and @ref
+        ohai "Checking out #{@spec} #{@ref}"
+        case @spec
+        when :branch
+          safe_system 'git', 'checkout', '-b', @ref, "origin/#{@ref}"
+        when :tag
+          safe_system 'git', 'checkout', @ref
+        end
+      end
       # http://stackoverflow.com/questions/160608/how-to-do-a-git-export-like-svn-export
       safe_system 'git', 'checkout-index', '-af', "--prefix=#{dst}/"
     end
@@ -200,7 +216,14 @@ class MercurialDownloadStrategy <AbstractDownloadStrategy
   def stage
     dst=Dir.getwd
     Dir.chdir @clone do
-      safe_system 'hg', 'archive', '-y', '-t', 'files', dst
+      if @spec and @ref
+        ohai "Checking out #{@spec} #{@ref}"
+        Dir.chdir @clone do
+          safe_system 'hg', 'archive', '-y', '-r', @ref, '-t', 'files', dst
+        end
+      else
+        safe_system 'hg', 'archive', '-y', '-t', 'files', dst
+      end
     end
   end
 end
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 6fa22feae..5d022d098 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -46,6 +46,7 @@ class Formula
   def initialize name='__UNKNOWN__'
     set_instance_variable 'url'
     set_instance_variable 'head'
+    set_instance_variable 'specs'
 
     if @head and (not @url or ARGV.flag? '--HEAD')
       @url=@head
@@ -85,7 +86,7 @@ class Formula
     self.class.path name
   end
 
-  attr_reader :url, :version, :homepage, :name
+  attr_reader :url, :version, :homepage, :name, :specs
 
   def bin; prefix+'bin' end
   def sbin; prefix+'sbin' end
@@ -281,7 +282,7 @@ private
   end
 
   def stage
-    ds=download_strategy.new url, name, version
+    ds=download_strategy.new url, name, version, specs
     HOMEBREW_CACHE.mkpath
     dl=ds.fetch
     verify_download_integrity dl if dl.kind_of? Pathname
@@ -380,7 +381,14 @@ private
       end
     end
 
-    attr_rw :url, :version, :homepage, :head, :deps, *CHECKSUM_TYPES
+    attr_rw :url, :version, :homepage, :specs, :deps, *CHECKSUM_TYPES
+
+    def head val=nil, specs=nil
+      if specs
+        @specs = specs
+      end
+      val.nil? ? @head : @head = val
+    end
 
     def depends_on name, *args
       @deps ||= []