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
