Update documentation to change "GIT" which was a poor-man's small
caps to "Git".  The latter was the intended spelling.

Also change "git" spelled in all-lowercase to "Git" when it refers
to the system as the whole or the concept it embodies, as opposed to
the command the end users would type.

* ta/doc-no-small-caps:
  Documentation: StGit is the right spelling, not StGIT
  Documentation: describe the "repository" in repository-layout
  Documentation: add a description for 'gitfile' to glossary
  Documentation: do not use undefined terms git-dir and git-file
  Documentation: the name of the system is 'Git', not 'git'
  Documentation: avoid poor-man's small caps GIT