pre-commit: Fix shellcheck warnings.
pre-commit: Fix shellcheck warnings.

In tools/pre-commit line 18:
if [ -z "$VIRTUAL_ENV" ] && `which vagrant > /dev/null` && [ -e .vagrant ]; then
                            ^-- SC2092: Remove backticks to avoid executing output.
                            ^-- SC2006: Use $(..) instead of legacy `..`.
                             ^-- SC2230: which is non-standard. Use builtin 'command -v' instead.

In tools/pre-commit line 23:
    ./tools/lint --no-gitlint --force $changed_files || true
                                      ^-- SC2086: Double quote to prevent globbing and word splitting.

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

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