BF: fix and test is_function

See: https://github.com/matthew-brett/multibuild/issues/166

Thanks to Andrew Murray for pointing out this bug.

common_utils.sh

@@ -88,7 +88,7 @@ function strip_ver_suffix {
 function is_function {
     # Echo "true" if input argument string is a function
     # Allow errors during "set -e" blocks.
-    (set +e; echo $($(declare -Ff "$1") > /dev/null && echo true))
+    (set +e; $(declare -Ff "$1" > /dev/null) && echo true)
 }
 
 function gh-clone {

tests/test_common_utils.sh

@@ -27,6 +27,16 @@
 bar=baz
 [ "$(is_function bar)" == "" ] || ingest "is_function bar"
 
+# Check function is not run in is_function. Thanks to Andrew Murray.
+function rmfile {
+    rm testfile
+}
+
+touch testfile
+[ "$(is_function rmfile)" == "true" ] || ingest "is_function rmfile"
+[ -f testfile ] || ingest "testfile removed during isfunction check"
+rm testfile
+
 rm_mkdir tmp_dir
 [ -d tmp_dir ] || ingest "tmp_dir does not exist"
 touch tmp_dir/afile