github: contribution: refactor diff command

[kamilsk]

Motivation: it contains invalid logic and showed problem after refactoring after 1522818.

$ maintainer github contribution diff --base=/tmp/snap.01.2013.json --head=/tmp/snap.02.2013.json
 Day / Week                  #46             #48             #49           #50    
---------------------- --------------- --------------- --------------- -----------
 Sunday                       -               -               -             -     
 Monday                       -               -               -             -     
 Tuesday                      -               -               -             -     
 Wednesday                   -4               -              -1             -     
 Thursday                     -               -               -            -1     
 Friday                       -              -2               -             -     
 Saturday                     -               -               -             -     
---------------------- --------------- --------------- --------------- -----------
 The diff between head{"/tmp/snap.02.2013.json"} → base{"/tmp/snap.01.2013.json"}

must be + instead of -.

Also, --base=/tmp/snap.01.2013.json --head=/tmp/snap.02.2013.json should be args, not params. See diff as reference, https://linuxize.com/post/diff-command-in-linux/.