User Manual: Write "Git" instead of "GIT"

User Manual: Write "Git" instead of "GIT"

In the Table of Contents, there is a notable inconsistency:
first there is "GIT Glossary", followed by "Git Quick Reference"
on the very next line.

Running "grep -c" on user-manual.txt, I find 780 occurrrences of
"git", 37 occurrences of "Git", and 9 occurrences of "GIT".
In general, "git" is the preferred spelling, except at the
beginning of a sentence.

Therefore, change "GIT Glossary" to "Git Glossary" for consistency
with the rest of the document.

Looking at the other eight occurrences of "GIT" I found one other
occurrence that should be changed:

* The mention of "StGIT". Looking at the web pages for "Stacked Git"
  at http://www.procode.org/stgit, I only saw the spelling "StGit",
  except in http://wiki.procode.org/cgi-bin/wiki.cgi/StGIT_Tutorial,
  but that page was last updated in 2006.

The other seven occurrences should not be changed:

* Three occurrences were in the output of 'git show-branch' run
  on the git.git repository.

* One occurrence was in the output of 'git cat-file'.

* One occurrence was as part of the file name "GIT-VERSION-GEN".

* Two occurrences were in comments in scripts quoted in a description
  of Tony Luck's workflow.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>