SVN
The development tree is held in SVN on Sourceforge. There are directories called "vips7" and "nip2".
You can browse the SVN repository. This will let you look at individual files and see who is changing what.
You need an svn client to get a local copy of the source. Sourceforge have a page about getting one.
vips7 and nip2 both have a trunk where current hacking is going on and stable branches which have the code for released, stable versions. Choose what you want and enter something like:
svn co https://vips.svn.sourceforge.net/svnroot/vips/vips7/trunk
to get the latest hacking on vips7, or perhaps
svn co https://vips.svn.sourceforge.net/svnroot/vips/nip2/branches/nip2-7.12
To get the source for the 7.12 stable version for nip2.
See sourceforge's page about svn, but try things like:
svn add myfile
svn delete old_stuff
svn copy thing copy_of_thing
svn move old_name new_name
svn update
When you're done, commit your changes back again:
svn commit --message "Fixed a bug" --username=MYUSERNAME
where MYUSERNAME is your Sourceforge ID. You'll be prompted for a password the first time you do this. You don't need to supply the username for subsequent commits.