mailmap: Add mailmap entry for Yash RE.
mailmap: Add mailmap entry for Yash RE.

This corrects the shortlog for the dozen or so commits had been made
with the wrong email ID for Yash RE and also unifies on "Yash RE"
rather than "YashRE42" or "Yash Rathore".

andersk/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip