feat(tools): support Sapling in code formatter

Some tools, such as tools/format, ask Git for a list of files to operate on.
If a contributor is not using Git, these tools fail.

Support the Sapling VCS in tools/format by falling back to 'sl files' if
'git ls-files' fails.

Author: strager

tools/files.sh

@@ -4,8 +4,19 @@
 # This file is a library used by other script. Do not execute this file
 # directly.
 
+find_repo_files() {
+  if git ls-files --cached --exclude-standard -- "${@}" 2>/dev/null; then
+    return
+  fi
+  if sl files "${@}" 2>/dev/null; then
+    return
+  fi
+  printf "error: could not run 'git ls-files' or 'sl files'\n'" >&2
+  return 1
+}
+
 find_non_vendor_files() {
-  git ls-files --cached --exclude-standard -- "${@}" \
+  find_repo_files "${@}" \
     | { grep -v '^vendor/.*/' || true ; } \
     | {
       while read path ; do