git: Set fetch.prune globally

Automatically delete remote tracking local branches that have been
removed from remote. This makes pruning references part of our
normal work flow.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

git/.gitconfig

@@ -12,6 +12,8 @@
 	smtpserver = /usr/bin/msmtp
 [http]
 	sslVerify = false
+[fetch]
+	prune = true
 [merge]
 	tool = vimdiff
 	conflictstyle = diff3