branch: add --dry-run for --prune-merged

With --dry-run, --prune-merged prints the local branches it would
delete, one "Would delete branch <name>" line per candidate, and
exits without touching any ref.

The @{push}-vs-@{upstream} and unmerged filtering still applies,
so the dry-run output is exactly the set that the live run would
delete.

--dry-run is only meaningful in combination with --prune-merged
and is rejected otherwise.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.adoc

@@ -25,7 +25,7 @@ git branch (-m|-M) [<old-branch>] <new-branch>
 git branch (-c|-C) [<old-branch>] <new-branch>
 git branch (-d|-D) [-r] <branch-name>...
 git branch --edit-description [<branch-name>]
-git branch (--prune-merged <branch>)...
+git branch [--dry-run] (--prune-merged <branch>)...
 
 DESCRIPTION
 -----------
@@ -230,6 +230,12 @@ Branches refused by the "fully merged" safety check are listed as
 warnings and skipped; pass them to `git branch -D` explicitly if
 you want them gone.
 
+`--dry-run`::
+	With `--prune-merged`, print which branches would be
+	deleted and exit without touching any ref.  Useful for
+	sanity-checking a wide pattern like `'origin/*'` before
+	committing to the deletion.
+
 `-v`::
 `-vv`::
 `--verbose`::

builtin/branch.c

@@ -860,7 +860,7 @@ static void collect_forked_set(const struct string_list *upstreams,
 }
 
 static int prune_merged_branches(const struct string_list *upstreams,
-				 int quiet)
+				 int quiet, int dry_run)
 {
 	struct ref_store *refs = get_main_ref_store(the_repository);
 	struct string_list candidates = STRING_LIST_INIT_DUP;
@@ -917,7 +917,7 @@ static int prune_merged_branches(const struct string_list *upstreams,
 				      quiet,
 				      1, /* warn_only */
 				      1, /* no_head_fallback */
-				      0  /* dry_run */);
+				      dry_run);
 
 	strvec_clear(&deletable);
 	string_list_clear(&candidates, 0);
@@ -967,6 +967,7 @@ int cmd_branch(int argc,
 	    unset_upstream = 0, show_current = 0, edit_description = 0;
 	struct string_list forked_upstreams = STRING_LIST_INIT_DUP;
 	struct string_list prune_merged_upstreams = STRING_LIST_INIT_DUP;
+	int dry_run = 0;
 	const char *new_upstream = NULL;
 	int noncreate_actions = 0;
 	/* possible options */
@@ -1024,6 +1025,8 @@ int cmd_branch(int argc,
 			N_("list local branches whose upstream matches <branch> (repeatable)")),
 		OPT_STRING_LIST(0, "prune-merged", &prune_merged_upstreams, N_("branch"),
 			N_("delete local branches whose upstream matches <branch> and is merged (repeatable)")),
+		OPT_BOOL(0, "dry-run", &dry_run,
+			N_("with --prune-merged, only print which branches would be deleted")),
 		OPT__FORCE(&force, N_("force creation, move/rename, deletion"), PARSE_OPT_NOCOMPLETE),
 		OPT_MERGED(&filter, N_("print only branches that are merged")),
 		OPT_NO_MERGED(&filter, N_("print only branches that are not merged")),
@@ -1083,6 +1086,9 @@ int cmd_branch(int argc,
 	if (noncreate_actions > 1)
 		usage_with_options(builtin_branch_usage, options);
 
+	if (dry_run && !prune_merged_upstreams.nr)
+		die(_("--dry-run requires --prune-merged"));
+
 	if (recurse_submodules_explicit) {
 		if (!submodule_propagate_branches)
 			die(_("branch with --recurse-submodules can only be used if submodule.propagateBranches is enabled"));
@@ -1124,7 +1130,7 @@ int cmd_branch(int argc,
 		if (argc)
 			die(_("--prune-merged does not take positional arguments; "
 			      "repeat --prune-merged for each <branch>"));
-		ret = prune_merged_branches(&prune_merged_upstreams, quiet);
+		ret = prune_merged_branches(&prune_merged_upstreams, quiet, dry_run);
 		goto out;
 	} else if (show_current) {
 		print_current_branch_name();

t/t3200-branch.sh

@@ -2027,4 +2027,48 @@ test_expect_success 'branch -d still deletes a pruneMerged=false branch' '
 	test_must_fail git -C pm-optout-d rev-parse --verify refs/heads/one
 '
 
+test_expect_success '--prune-merged --dry-run lists but does not delete' '
+	test_when_finished "rm -rf pm-dry" &&
+	git clone pm-upstream pm-dry &&
+	git -C pm-dry remote add fork ../pm-fork &&
+	test_config -C pm-dry remote.pushDefault fork &&
+	test_config -C pm-dry push.default current &&
+	git -C pm-dry branch one one-commit &&
+	git -C pm-dry branch --set-upstream-to=origin/next one &&
+	git -C pm-dry branch two two-commit &&
+	git -C pm-dry branch --set-upstream-to=origin/next two &&
+
+	git -C pm-dry branch --dry-run --prune-merged "origin/*" >actual &&
+	test_grep "Would delete branch one " actual &&
+	test_grep "Would delete branch two " actual &&
+
+	git -C pm-dry rev-parse --verify refs/heads/one &&
+	git -C pm-dry rev-parse --verify refs/heads/two
+'
+
+test_expect_success '--prune-merged --dry-run only lists branches the live run would delete' '
+	test_when_finished "rm -rf pm-dry-mixed" &&
+	git clone pm-upstream pm-dry-mixed &&
+	git -C pm-dry-mixed remote add fork ../pm-fork &&
+	test_config -C pm-dry-mixed remote.pushDefault fork &&
+	test_config -C pm-dry-mixed push.default current &&
+	git -C pm-dry-mixed checkout -b wip origin/next &&
+	git -C pm-dry-mixed branch --set-upstream-to=origin/next wip &&
+	test_commit -C pm-dry-mixed local-only &&
+	git -C pm-dry-mixed checkout - &&
+	git -C pm-dry-mixed branch merged one-commit &&
+	git -C pm-dry-mixed branch --set-upstream-to=origin/next merged &&
+
+	git -C pm-dry-mixed branch --dry-run --prune-merged "origin/*" >out &&
+	test_grep "Would delete branch merged" out &&
+	test_grep ! "Would delete branch wip" out &&
+	git -C pm-dry-mixed rev-parse --verify refs/heads/wip &&
+	git -C pm-dry-mixed rev-parse --verify refs/heads/merged
+'
+
+test_expect_success '--dry-run without --prune-merged is rejected' '
+	test_must_fail git -C forked branch --dry-run 2>err &&
+	test_grep "requires --prune-merged" err
+'
+
 test_done