pull: pass --allow-unrelated-histories to "git merge"

The previous commit said:

    We could add the same option to "git pull" and have it passed
    through to underlying "git merge".  I do not have a fundamental
    opposition against such a feature, but this commit does not do
    so and instead leaves it as low-hanging fruit for others,
    because such a "two project merge" would be done after fetching
    the other project into some location in the working tree of an
    existing project and making sure how well they fit together, it
    is sufficient to allow a local merge without such an option
    pass-through from "git pull" to "git merge".

Prepare a patch to make it a reality, just in case it is needed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -11,6 +11,7 @@ SYNOPSIS
 [verse]
 'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
 	[-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
+	[--[no-]allow-unrelated-histories]
 	[--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
 'git merge' <msg> HEAD <commit>...
 'git merge' --abort
@@ -98,19 +99,6 @@ commit or stash your changes before running 'git merge'.
 'git merge --abort' is equivalent to 'git reset --merge' when
 `MERGE_HEAD` is present.
 
---allow-unrelated-histories::
-	By default, `git merge` command refuses to merge histories
-	that do not share a common ancestor.  This option can be
-	used to override this safety when merging histories of two
-	projects that started their lives independently.  As that is
-	a very rare occasion, no configuration variable to enable
-	this by default exists and will not be added, and the list
-	of options at the top of this documentation does not mention
-	this option.  Also `git pull` does not pass this option down
-	to `git merge` (instead, you `git fetch` first, examine what
-	you will be merging and then `git merge` locally with this
-	option).
-
 <commit>...::
 	Commits, usually other branch heads, to merge into our branch.
 	Specifying more than one commit will create a merge with

Documentation/merge-options.txt

@@ -114,3 +114,11 @@ ifndef::git-pull[]
 	reporting.
 
 endif::git-pull[]
+
+--allow-unrelated-histories::
+	By default, `git merge` command refuses to merge histories
+	that do not share a common ancestor.  This option can be
+	used to override this safety when merging histories of two
+	projects that started their lives independently. As that is
+	a very rare occasion, no configuration variable to enable
+	this by default exists and will not be added.

builtin/pull.c

@@ -86,6 +86,7 @@ static char *opt_verify_signatures;
 static struct argv_array opt_strategies = ARGV_ARRAY_INIT;
 static struct argv_array opt_strategy_opts = ARGV_ARRAY_INIT;
 static char *opt_gpg_sign;
+static int opt_allow_unrelated_histories;
 
 /* Options passed to git-fetch */
 static char *opt_all;
@@ -155,6 +156,9 @@ static struct option pull_options[] = {
 	OPT_PASSTHRU('S', "gpg-sign", &opt_gpg_sign, N_("key-id"),
 		N_("GPG sign commit"),
 		PARSE_OPT_OPTARG),
+	OPT_SET_INT(0, "allow-unrelated-histories",
+		    &opt_allow_unrelated_histories,
+		    N_("allow merging unrelated histories"), 1),
 
 	/* Options passed to git-fetch */
 	OPT_GROUP(N_("Options related to fetching")),
@@ -603,6 +607,8 @@ static int run_merge(void)
 	argv_array_pushv(&args, opt_strategy_opts.argv);
 	if (opt_gpg_sign)
 		argv_array_push(&args, opt_gpg_sign);
+	if (opt_allow_unrelated_histories > 0)
+		argv_array_push(&args, "--allow-unrelated-histories");
 
 	argv_array_push(&args, "FETCH_HEAD");
 	ret = run_command_v_opt(args.argv, RUN_GIT_CMD);

t/t5521-pull-options.sh

@@ -144,4 +144,25 @@ test_expect_success 'git pull --all --dry-run' '
 	)
 '
 
+test_expect_success 'git pull --allow-unrelated-histories' '
+	test_when_finished "rm -fr src dst" &&
+	git init src &&
+	(
+		cd src &&
+		test_commit one &&
+		test_commit two
+	) &&
+	git clone src dst &&
+	(
+		cd src &&
+		git checkout --orphan side HEAD^ &&
+		test_commit three
+	) &&
+	(
+		cd dst &&
+		test_must_fail git pull ../src side &&
+		git pull --allow-unrelated-histories ../src side
+	)
+'
+
 test_done