Suggest use of "git add file1 file2" when there is nothing to commit.

Suggest use of "git add file1 file2" when there is nothing to commit.

If a user modifies files and runs 'git commit' (without the very
useful -a option) and they have not yet updated the index they
are probably coming from another SCM-like tool which would perform
the same as 'git commit -a' in this case.  Showing the user their
current status and a final line of "nothing to commit" is not very
reassuring, as the user might believe that Git did not recognize
their files were modified.

Instead we can suggest as part of the 'nothing to commit' message
that the user invoke 'git add' to add files to their next commit.

Suggested by Andy Parkins' Git 'niggles' list
(<200612132237.10051.andyparkins@gmail.com>).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>