demos: redirect setup commands to /dev/null

Previously, we needed to remove this manually from screenshots.

demos/demo_juggle_conflicts.sh

@@ -3,16 +3,18 @@ set -euo pipefail
 . "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
-jj init --config-toml ui.allow-init-native=true
-echo "first" > file
-jj branch create first
-jj commit -m 'first' 
-echo "second" > file
-jj branch create second
-jj commit -m 'second' 
-echo "third" > file
-jj branch create third
-jj commit -m 'third' 
+(
+    jj init --config-toml ui.allow-init-native=true
+    echo "first" > file
+    jj branch create first
+    jj commit -m 'first'
+    echo "second" > file
+    jj branch create second
+    jj commit -m 'second'
+    echo "third" > file
+    jj branch create third
+    jj commit -m 'third'
+) >/dev/null
 
 comment "We are in a repo with three commits, all
 editing the same line:"

demos/demo_operation_log.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 . "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
-jj git clone https://github.com/octocat/Hello-World
+jj git clone https://github.com/octocat/Hello-World > /dev/null
 cd Hello-World
 
 comment "We are in the octocat/Hello-World repo.

demos/demo_resolve_conflicts.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 . "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
-jj git clone https://github.com/octocat/Hello-World
+jj git clone https://github.com/octocat/Hello-World > /dev/null
 cd Hello-World
 
 comment "We are on the master branch of the

demos/demo_working_copy.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 . "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
-jj git clone https://github.com/octocat/Hello-World
+jj git clone https://github.com/octocat/Hello-World > /dev/null
 cd Hello-World
 
 comment "We are in the octocat/Hello-World repo.