Fix #1428 - zstdgrep returned 1 on match and unmatch

- Use ZCAT for testing zstdgrep in case of non-install yet
- tests: Add file test for zstdgrep

programs/zstdgrep

@@ -22,8 +22,8 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 # THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-grep=grep
-zcat=zstdcat
+grep=${GREP:-grep}
+zcat=${ZCAT:-zstdcat}
 
 endofopts=0
 pattern_found=0
@@ -31,12 +31,13 @@ grep_args=""
 hyphen=0
 silent=0
 
-prg=$(basename "$0")
+prog=${0##*/}
 
 # handle being called 'zegrep' or 'zfgrep'
-case "${prg}" in
-    *zegrep) grep_args="-E";;
-    *zfgrep) grep_args="-F";;
+case $prog in
+    *egrep*) prog=zegrep; grep_args='-E';;
+    *fgrep*) prog=zfgrep; grep_args='-F';;
+    *)       prog=zstdgrep;;
 esac
 
 # skip all options and pass them on to grep taking care of options
@@ -47,7 +48,7 @@ while [ "$#" -gt 0 ] && [ "${endofopts}" -eq 0 ]; do
     # from GNU grep-2.5.1 -- keep in sync!
         -[ABCDXdefm])
             if [ "$#" -lt 2 ]; then
-                printf '%s: missing argument for %s flag\n' "${prg}" "$1" >&2
+                printf '%s: missing argument for %s flag\n' "${prog}" "$1" >&2
                 exit 1
             fi
             case "$1" in
@@ -94,7 +95,7 @@ if [ "${pattern_found}" -lt 1 ]; then
     elif [ "${hyphen}" -gt 0 ]; then
         pattern="-"
     else
-        printf '%s: missing pattern\n' "${prg}" >&2
+        printf '%s: missing pattern\n' "${prog}" >&2
         exit 1
     fi
 fi
@@ -113,16 +114,11 @@ else
     if [ "${silent}" -lt 1 ] && [ "$#" -gt 1 ]; then
         grep_args="-H ${grep_args}"
     fi
-    CUR_EXIT_CODE=0
-    EXIT_CODE=1
     set -f
     while [ "$#" -gt 0 ]; do
         # shellcheck disable=SC2086
         "${zcat}" -fq -- "$1" | "${grep}" --label="${1}" ${grep_args} -- "${pattern}" -
-        CUR_EXIT_CODE=$?
-        if [ "${CUR_EXIT_CODE}" -eq 0 ] && [ "${EXIT_CODE}" -ne 1 ]; then
-            EXIT_CODE=0
-        fi
+        [ "$?" -ne 0 ] && EXIT_CODE=1
         shift
     done
     set +f