bash: use -prune in find (#127)

Use -prune to handle excluded paths, rather than simply ! -path, which
will still visit all subdirectories and files in the excluded paths.

This is more efficient, but more importantly avoids visiting files which
may not be readable by the user, which causes `find` fail, and causes
buildifier to fail as we are using `-o pipefail`.

buildifier/factory.bzl

diff --git a/buildifier/factory.bzl b/buildifier/factory.bzl
index 6a8098c..40978bc 100644
--- a/buildifier/factory.bzl
+++ b/buildifier/factory.bzl
@@ -133,8 +133,8 @@
     if ctx.attr.exclude_patterns:
         if test_rule and not ctx.attr.no_sandbox:
             fail("Cannot use 'exclude_patterns' in a test rule without 'no_sandbox'")
-        exclude_patterns = ["\\! -path %s" % shell.quote(pattern) for pattern in ctx.attr.exclude_patterns]
-        exclude_patterns_str = " ".join(exclude_patterns)
+        exclude_patterns = ["-path %s" % shell.quote(pattern) for pattern in ctx.attr.exclude_patterns]
+        exclude_patterns_str = " -o ".join(exclude_patterns) + " -prune -o "
 
     workspace = ""
     if test_rule and ctx.attr.no_sandbox:

runner.bash.template

diff --git a/runner.bash.template b/runner.bash.template
index a23f11b..e181354 100644
--- a/runner.bash.template
+++ b/runner.bash.template
@@ -31,8 +31,8 @@
 
 # Run buildifier on all starlark files
 find . \
-  -type "${FIND_FILE_TYPE:-f}" \
   @@EXCLUDE_PATTERNS@@ \
+  -type "${FIND_FILE_TYPE:-f}" \
   \( -name '*.bzl' \
     -o -name '*.sky' \
     -o -name '*.bazel' \