test: Work around ESLint ignored-file error wart

This resolves an issue where `tools/test` would fail with an ESLint
error because you'd touched a file that we'd told ESLint to ignore.

Fixes: #5081

.eslintignore

@@ -1,11 +1,5 @@
-# If you run `tools/test` when you've modified a file that's ignored
-# here, you might get a warning like this:
-#   0:0  warning  File ignored because of a matching ignore pattern. […]
-#
-# Don't worry about that warning; it won't appear in CI, and it won't
-# appear on future `tools/test` runs when not editing these files.
-# For more discussion, see:
-#   https://github.com/zulip/zulip-mobile/pull/4430#issuecomment-767297445
+# This should be the only .eslintignore file in our tree.
+# See apply_eslintignore in tools/test.
 
 # These are purely type definitions, no runtime code.  Most of them
 # are third-party code, too, so naturally don't match our style.

tools/test

@@ -176,9 +176,41 @@ run_native() {
     fi
 }
 
+# Filter out filenames excluded by our .eslintignore.
+#
+# Filenames are expected as arguments, and printed, leaving out those that
+# should be ignored.
+#
+# (This is a workaround: the ideal solution would be for ESLint to accept an
+# option saying to apply its ignore rules and to not worry if that filters
+# out some or all of the files we named, and then we wouldn't need this.)
+apply_eslintignore() {
+    # This uses only the .eslintignore file at the root.  It relies on us
+    # having no other sources of ESLint file ignores: other .eslintignore
+    # files, `ignorePatterns` in the eslintrc, etc.
+    #
+    # But if we do, the worst case is that we'll reintroduce #5081: the
+    # interactive `tools/test` (never CI) will fail when one of those
+    # ignored files is touched, because of an awkward CLI choice in ESLint.
+
+    # ESLint ignore files are documented as having the gitignore syntax:
+    #   https://eslint.org/docs/user-guide/configuring/ignoring-code
+    # So we ask Git to interpret the file for us.  The main limitation is
+    # that while we can tell it one file to apply, we can't give it several,
+    # or tell it to look for .eslintignore files throughout the tree.
+    #
+    # The `git check-ignore` command wants to tell us what files have been
+    # excluded, not included.  With `-nv` it gives us both and we can
+    # extract which is which.
+    (( $# )) || return 0  # if no arguments, no output
+    git -c core.excludesFile=.eslintignore check-ignore --no-index -nv "$@" \
+    | perl -lne 'print if (s/^::\t//)'
+}
+
 run_lint() {
-    (( $# )) || return 0
-    eslint ${fix:+--fix} --max-warnings=0 "$@"
+    files=( $(apply_eslintignore "$@") )
+    (( ${#files[@]} )) || return 0
+    eslint ${fix:+--fix} --max-warnings=0 "${files[@]}"
 }
 
 check_node() {