script: fix credits.sh showing 0 lines changed for mapped emails

script: fix credits.sh showing 0 lines changed for mapped emails

script was showing 0 for several contributors because the commit Author:
did not match their mapped address:
```
Number of lines added and removed, by authors
...
   306	     0 Radoslaw Zarzynski <rzarzynski@redhat.com>
   307	     0 Mike Perez <miperez@redhat.com>
   308	     0 Michael J. Kidd <linuxkidd@redhat.com>
   309	     0 Lukas Mayer <lmayer@wind.gmbh>
   310	     0 Luis Henriques <lhenriques@suse.com>
   311	     0 Kyle McGough <kmcgough@digitalocean.com>
   312	     0 João Eduardo Luís <joao@suse.de>
   313	     0 JinyongHa <jy200.ha@samsung.com>
   314	     0 Ilya Dryomov <idryomov@redhat.com>
```

in the case of Ilya and Radoslaw:
```
$ git log --no-merges --pretty='%ae' v17.2.6..v18.2.0 | sed -e "s/'/ /g" | sort -u | grep -e idryomov -e rzarzyns
idryomov@gmail.com
rzarzyns@redhat.com
```

this output gets fed into `git log --numstat --author="$mail"` to fetch
the stats, but that command maps idryomov@gmail.com -> idryomov@redhat.com
and rzarzyns@redhat.com -> rzarzynski@redhat.com so no commits matched.
disabling the mapping for this command with --no-mailmap allows it to
fetch their stats correctly:
```
    14   11809 Radoslaw Zarzynski <rzarzynski@redhat.com>
    16   10051 Ilya Dryomov <idryomov@redhat.com>
```

Signed-off-by: Casey Bodley <cbodley@redhat.com>