Here is the final command I did (thanks to answer by Gary Fixler and a forum) :
grep "^Subject:" his_last_3_commits.patch | sed "s/'/\\\\\\\\\\\'/g" | xargs --max-args=1 -I% php -r "echo iconv_mime_decode('%', 0, 'UTF-8');echo \"\n\";"
PS : if someone knows a git
command which does that, don't hesistate to answer here.