rbinstall.rb: fix for non-working directory

* tool/rbinstall.rb (Gem::Specification.last_date): skip if failed to
  get info from VCS, for example, in the case circumstance sharing working
  directory with another machine and run vcs tools cannot work.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43624 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -579,7 +579,8 @@ module Gem
 
     def self.last_date(path)
       return unless $vcs
-      return unless time = $vcs.get_revisions(path)[2]
+      time = $vcs.get_revisions(path)[2] rescue return
+      return unless time
       time.strftime("%Y-%m-%d")
     end
 

tool/vcs.rb

@@ -43,8 +43,8 @@ class VCS
   def get_revisions(path)
     path = relative_to(path)
     last, changed, modified, *rest = Dir.chdir(@srcdir) {self.class.get_revisions(path)}
-    last or raise "last revision not found"
-    changed or raise "changed revision not found"
+    last or raise VCS::NotFoundError, "last revision not found"
+    changed or raise VCS::NotFoundError, "changed revision not found"
     modified &&= Time.parse(modified)
     return last, changed, modified, *rest
   end