deploy-branch: Fix shellcheck warnings.
deploy-branch: Fix shellcheck warnings.

In tools/deploy-branch line 17:
[ $? -ne 0 ] && error_out "Unknown branch: $branch"
  ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

In tools/deploy-branch line 23:
    if [ $? -eq 0 ]; then
         ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

In tools/deploy-branch line 35:
[ $? -ne 0 ] && error_out "Rebase onto origin/master failed"
  ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

In tools/deploy-branch line 39:
[ $? -ne 0 ] && error_out "Push of master to origin/master failed"
  ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

timabbott/zulip,timabbott/zulip,rishig/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,tommyip/zulip,tommyip/zulip,jackrzhang/zulip,rht/zulip,rishig/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,tommyip/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,rishig/zulip,timabbott/zulip,hackerkid/zulip,jackrzhang/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,rht/zulip,tommyip/zulip,tommyip/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,punchagan/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,tommyip/zulip,synicalsyntax/zulip,punchagan/zulip,brainwane/zulip,showell/zulip,synicalsyntax/zulip,jackrzhang/zulip,brainwane/zulip,timabbott/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,dhcrzf/zulip,timabbott/zulip,brainwane/zulip,jackrzhang/zulip,zulip/zulip,timabbott/zulip,shubhamdhama/zulip,dhcrzf/zulip,punchagan/zulip,andersk/zulip,rishig/zulip,brainwane/zulip,eeshangarg/zulip,jackrzhang/zulip,rishig/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,dhcrzf/zulip,dhcrzf/zulip,hackerkid/zulip,eeshangarg/zulip,jackrzhang/zulip,kou/zulip,rht/zulip,timabbott/zulip,rishig/zulip,andersk/zulip,synicalsyntax/zulip,dhcrzf/zulip,zulip/zulip,dhcrzf/zulip,zulip/zulip,showell/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,rishig/zulip,brainwane/zulip,andersk/zulip,tommyip/zulip,kou/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,synicalsyntax/zulip,punchagan/zulip,hackerkid/zulip,jackrzhang/zulip