Blogposts: extract metadata from git

[rien]

For example: date created, last edited, author email & username.
This data is probably stored in the git metadata, so we don't have to fill in this information by hand.

[JensTimmerman]

related to https://groups.google.com/g/nanoc/c/tYR2FtP7p5o/m/4fI1hAg0rBUJ

[JensTimmerman]

fetching the
modification date of a single file is antithetical to the way git works
since git does not track per-file history. Fetching the date of a single
file is a Slow Operation because it requires scanning global history
looking for a commit in which the desired file was changed.

[JensTimmerman]

it's pretty easy to create more functions like the one used in #409 to get different metadata from git:

  def gitmtime
     # find file last modification time
     filepath=@item[:content_filename]
     str=`git log -1 --format=%cd --date=short -- #{filepath}`
     return Date.parse(str)
 end

e.g. to get the commiters name use --format='%cn' and to get commiters email use --format='%ce'
e.g. to get the authors name use --format='%an' and to get authors email use --format='%ae'