Simplify check for git working tree in 3.0

Sending the entire log to /dev/null is a rather expensive way of checking whether
we are inside a git working tree. Use `git rev-parse --is-inside-work-tree` instead.

(cherry picked from commit 1e3f932)

Author: mspncp

testssl.sh

@@ -122,7 +122,7 @@ declare -r SWCONTACT="dirk aet testssl dot sh"
 grep -E -q "dev|rc|beta" <<< "$VERSION" && \
      SWURL="https://testssl.sh/dev/" ||
      SWURL="https://testssl.sh/"
-if git log &>/dev/null; then
+if git rev-parse --is-inside-work-tree &>/dev/null; then
      declare -r GIT_REL="$(git log --format='%h %ci' -1 2>/dev/null | awk '{ print $1" "$2" "$3 }')"
      declare -r GIT_REL_SHORT="$(git log --format='%h %ci' -1 2>/dev/null | awk '{ print $1 }')"
      declare -r REL_DATE="$(git log --format='%h %ci' -1 2>/dev/null | awk '{ print $2 }')"