Add obj to gitignore

[su8]

Exclude the obj/* path in .gitignore being pushed accidentally.

[su8]

Also diff3 tries to install to as /usr/libexec resulting in permission error:

===> usr.bin/diff3
test -d "/usr/libexec" || \
        /usr/bin/install -c -d -m 755 /usr/libexec/
/usr/bin/install -c diff3prog /usr/libexec/
/usr/bin/install: cannot create regular file '/usr/libexec/diff3prog': Permission denied

[Duncaen]

Thanks, working on it, there are a few other places where paths are hardcoded, going to fix them all now.