Add GitInfo

This commit adds a `GitInfo` object to `Page` if `EnableGitInfo` is set.

It then also sets `Lastmod` for the given `Page` to the author date provided by Git.

The Git integrations should be fairly performant, but it adds "some time" to the build, somewhat depending on the Git history size.

If you want, you can run without during development and turn it on when deploying to the live server: `hugo --enableGitInfo`.

Fixes #2102

commands/hugo.go

@@ -133,6 +133,7 @@ var (
 	canonifyURLs          bool
 	cleanDestination      bool
 	enableRobotsTXT       bool
+	enableGitInfo         bool
 	disable404            bool
 	disableRSS            bool
 	disableSitemap        bool
@@ -237,6 +238,7 @@ func initHugoBuildCommonFlags(cmd *cobra.Command) {
 	cmd.Flags().BoolVar(&uglyURLs, "uglyURLs", false, "if true, use /filename.html instead of /filename/")
 	cmd.Flags().BoolVar(&canonifyURLs, "canonifyURLs", false, "if true, all relative URLs will be canonicalized using baseURL")
 	cmd.Flags().StringVarP(&baseURL, "baseURL", "b", "", "hostname (and path) to the root, e.g. http://spf13.com/")
+	cmd.Flags().BoolVar(&enableGitInfo, "enableGitInfo", false, "Add Git revision, date and author info to the pages")
 
 	cmd.Flags().BoolVar(&nitro.AnalysisOn, "stepAnalysis", false, "display memory and timing of different steps of the program")
 	cmd.Flags().BoolVar(&pluralizeListTitles, "pluralizeListTitles", true, "Pluralize titles in lists using inflect")
@@ -319,6 +321,9 @@ func InitializeConfig(subCmdVs ...*cobra.Command) error {
 		if flagChanged(cmdV.Flags(), "enableRobotsTXT") {
 			viper.Set("enableRobotsTXT", enableRobotsTXT)
 		}
+		if flagChanged(cmdV.Flags(), "enableGitInfo") {
+			viper.Set("enableGitInfo", enableGitInfo)
+		}
 		if flagChanged(cmdV.Flags(), "pluralizeListTitles") {
 			viper.Set("pluralizeListTitles", pluralizeListTitles)
 		}