Prevent git commands if project is not a git repo

Prior to this commit, git commands (such as the command to get
`remote.origin.url`) would always be run if git was installed.  This
caused "fatal: not a git repository" to be printed to `stderr` when the
project was not a git repository.

This commit modifies `SDoc::Helpers::Git#git?` such that git commands
will only be run if git is installed _and_ the project is a git
repository.

lib/sdoc/helpers/git.rb

@@ -9,11 +9,11 @@ def github?
     @options.github && git? && github_repository
   end
 
-  attr_writer :git_bin_path
+  attr_writer :git_repo_path
 
   def git?
-    @git_bin_path ||= `sh -c 'command -v git'`.chomp
-    !@git_bin_path.empty?
+    @git_repo_path ||= Dir.chdir(@options.root) { `git rev-parse --show-toplevel 2> /dev/null`.chomp }
+    !@git_repo_path.empty?
   end
 
   def git_command(command)

spec/helpers_spec.rb

@@ -12,13 +12,29 @@
     @helpers.options = RDoc::Options.new
   end
 
+  describe "#git?" do
+    it "returns false when git is not installed" do
+      with_env("PATH" => "") do
+        _(@helpers.git?).must_equal false
+      end
+    end
+
+    it "returns false when project is not a git repository" do
+      Dir.mktmpdir do |dir|
+        @helpers.options.root = dir
+
+        _(@helpers.git?).must_equal false
+      end
+    end
+  end
+
   describe "#github_url" do
     before :each do
       @helpers.options.github = true
     end
 
     it "returns the URL for a given path in the project's GitHub repository at the current SHA1" do
-      @helpers.git_bin_path = "path/to/git"
+      @helpers.git_repo_path = "path/to/repo"
       @helpers.git_origin_url = "[email protected]:user/repo.git"
       @helpers.git_head_sha1 = "1337c0d3"
 
@@ -51,8 +67,8 @@
       _(@helpers.github_url("foo/bar/qux.rb")).must_be_nil
     end
 
-    it "returns nil when git is not installed" do
-      @helpers.git_bin_path = ""
+    it "returns nil when git is not installed or project is not a git repository" do
+      @helpers.git_repo_path = ""
 
       _(@helpers.github_url("foo/bar/qux.rb")).must_be_nil
     end
@@ -306,7 +322,7 @@ module Foo; module Bar; module Qux; end; end; end
 
   describe "#project_git_head" do
     it "returns the branch name and abbreviated SHA1 of the most recent commit in HEAD" do
-      @helpers.git_bin_path = "path/to/git"
+      @helpers.git_repo_path = "path/to/repo"
       @helpers.git_head_branch = "1-0-stable"
       @helpers.git_head_sha1 = "1337c0d3d00d" * 3
 
@@ -317,8 +333,8 @@ module Foo; module Bar; module Qux; end; end; end
       _(@helpers.project_git_head).must_match %r"\A.+@[[:xdigit:]]{12}\z"
     end
 
-    it "returns nil when git is not installed" do
-      @helpers.git_bin_path = ""
+    it "returns nil when git is not installed or project is not a git repository" do
+      @helpers.git_repo_path = ""
 
       _(@helpers.project_git_head).must_be_nil
     end
@@ -367,7 +383,7 @@ module Foo; module Bar; module Qux; end; end; end
 
   describe "#og_modified_time" do
     it "returns the commit time of the most recent commit in HEAD" do
-      @helpers.git_bin_path = "path/to/git"
+      @helpers.git_repo_path = "path/to/repo"
       @helpers.git_head_timestamp = "1999-12-31T12:34:56Z"
 
       _(@helpers.og_modified_time).must_equal "1999-12-31T12:34:56Z"
@@ -378,8 +394,8 @@ module Foo; module Bar; module Qux; end; end; end
         must_match %r"\A\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[-+]\d{2}:\d{2}\z"
     end
 
-    it "returns nil when git is not installed" do
-      @helpers.git_bin_path = ""
+    it "returns nil when git is not installed or project is not a git repository" do
+      @helpers.git_repo_path = ""
 
       _(@helpers.og_modified_time).must_be_nil
     end