git.txt: correct stale 'GIT_EXTERNAL_DIFF' description

git.txt: correct stale 'GIT_EXTERNAL_DIFF' description

In fde97d8ac6 (Update documentation to remove incorrect GIT_DIFF_OPTS
example., 2006-11-27), the description of the 'GIT_EXTERNAL_DIFF'
variable was moved from 'diff-format.txt' to 'git.txt', and the
documentation was updated to remove a 'diff(1)' invocation since Git did
not use an external diff program anymore by default.

However, the description of 'GIT_EXTERNAL_DIFF' still mentions "instead
of the diff invocation described above", which is confusing.

Correct that outdated sentence.

Also, link to git(1) in 'diff-generate-patch.txt' when GIT_DIFF_OPTS and
GIT_EXTERNAL_DIFF are mentioned, so that users can easily know what
these variables are about.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>