Revert "graph.c: mark private file-scope symbols as static"

This reverts commit ba35480439d05b8f6cca50527072194fe3278bbb.

CGit uses these symbols to output the correct HTML around graph
elements.  Making these symbols private means that CGit cannot be
updated to use Git 1.8.0 or newer, so let's not do that.

On top of the revert, also add comments so that we avoid reintroducing
this problem in the future and suggest to those modifying this API
that they might want to discuss it with the CGit developers.

Signed-off-by: John Keeping <john@keeping.me.uk>