Clean up and relnote shebangless script support

This adds a test for shebangless support from #7802, cleans up some of
its tricks, and includes it in the changelog.

tests/checks/noshebang.fish

@@ -0,0 +1,66 @@
+# RUN: %fish %s
+
+# Test for shebangless scripts - see 7802.
+
+set testdir (mktemp -d)
+cd $testdir
+
+touch file
+chmod a+x file
+
+function runfile
+    # Run our file twice, printing status.
+    # Arguments are passed to exercise the re-execve code paths; they have no other effect.
+    set -g fish_use_posix_spawn 0
+    ./file arg1 arg2 arg3
+    echo $status
+
+    set -g fish_use_posix_spawn 1
+    ./file arg1 arg2 arg3 arg4 arg5
+    echo $status
+end
+
+# Empty executable files are 'true'.
+true >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# Files without NUL are 'true' as well.
+echo -e -n '#COMMENT\n#COMMENT' >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# On to NUL bytes.
+# The heuristic is that there must be a line containing a lowercase letter before the first NUL byte.
+echo -n -e 'true\n\x00' >file
+runfile
+#CHECK: 0
+#CHECK: 0
+
+# Doesn't meet our heuristic as there is no newline.
+echo -n -e 'true\x00' >file
+runfile
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+# Doesn't meet our heuristic as there is no lowercase before newline.
+echo -n -e 'NOPE\n\x00' >file
+runfile
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}
+
+#CHECK: 125
+#CHECKERR: Failed {{.*}}
+#CHECKERR: exec: {{.*}}
+#CHECKERR: {{.*}}