Add format command to force unconditional formatting (#1186)

This allows users of buildozer to format files without also having
buildifier installed. This was possible previously by making a trivial
edit and reverting it, but a dedicated command is cleaner.

Will be used as part of the upcoming `bazel mod tidy` command.

buildozer/README.md

@@ -131,6 +131,8 @@ Buildozer supports the following commands(`'command args'`):
   * `dict_remove <attr> <key(s)>`:  Deletes the key for the dict attribute `attr`.
   * `dict_list_add <attr> <key> <value(s)>`:  Adds value(s) to the list in the
     dict attribute `attr`.
+  * `format`: Force formatting of all files, even if they were not changed by
+    other commands.
 
 Here, `<attr>` represents an attribute (being `add`ed/`rename`d/`delete`d etc.),
 e.g.: `srcs`, `<value(s)>` represents values of the attribute and so on.

buildozer/buildozer_test.sh

@@ -2299,4 +2299,22 @@ EOF
   diff -u MODULE.bazel.expected MODULE.bazel || fail "Output didn't match"
 }
 
+function test_format() {
+  cat > MODULE.bazel <<EOF
+module(
+  name = "foo", version = "0.27.0",
+)
+EOF
+
+  cat > MODULE.bazel.expected <<EOF
+module(
+    name = "foo",
+    version = "0.27.0",
+)
+EOF
+
+  $buildozer 'format' //MODULE.bazel:all
+  diff -u MODULE.bazel.expected MODULE.bazel || fail "Output didn't match"
+}
+
 run_suite "buildozer tests"

edit/buildozer.go

@@ -813,6 +813,11 @@ func cmdImplUseRepo(env CmdEnvironment, mode string) (*build.File, error) {
 	return env.File, nil
 }
 
+func cmdFormat(opts *Options, env CmdEnvironment) (*build.File, error) {
+	// Force formatting by not returning a nil *build.File.
+	return env.File, nil
+}
+
 func isExtensionLabel(arg string) bool {
 	// Labels referencing extensions are either absolute or repo-absolute. Repository names are not
 	// allowed to contain "@" or "/".
@@ -868,6 +873,7 @@ var AllCommands = map[string]CommandInfo{
 	"dict_list_add":     {cmdDictListAdd, true, 3, -1, "<attr> <key> <value(s)>"},
 	"use_repo_add":      {cmdUseRepoAdd, false, 2, -1, "([dev] <extension .bzl file> <extension name>|<use_extension variable name>) <repo(s)>"},
 	"use_repo_remove":   {cmdUseRepoRemove, false, 2, -1, "([dev] <extension .bzl file> <extension name>|<use_extension variable name>) <repo(s)>"},
+	"format":            {cmdFormat, false, 0, 0, ""},
 }
 
 var readonlyCommands = map[string]bool{