upgrade-zulip-from-git: Run git fetch with --prune.
upgrade-zulip-from-git: Run git fetch with --prune.

This prevents upgrading to an obsolete version of a branch that has
been deleted or renamed.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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