Documentation/git: drop outdated Cogito reference

Cogito hasn't been maintained since late 2006, so drop the reference
to it. The warning that SCMS front-ends might override listed
environment variables, however, may still be valuable, so keep it but
generalize the wording.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sunshineco

Documentation/git.txt

@@ -744,7 +744,7 @@ The Git Repository
 ~~~~~~~~~~~~~~~~~~
 These environment variables apply to 'all' core Git commands. Nb: it
 is worth noting that they may be used/overridden by SCMS sitting above
-Git so take care if using Cogito etc.
+Git so take care if using a foreign front-end.
 
 'GIT_INDEX_FILE'::
 	This environment allows the specification of an alternate