[PATCH v2 02/12] fmt-merge-msg: introduce a way to override the main branch name

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

There is a growing number of projects and companies desiring to change
the main branch name of their repositories (see e.g.
https://twitter.com/mislav/status/1270388510684598272 for background on
this).

However, there are a couple of hard-coded spots in Git's source code
that make this endeavor harder than necessary. For example, when
formatting the commit message for merge commits, Git appends "into
<branch-name>" unless the current branch is the `master` branch.

Clearly, this is not what one wants when already having gone through all
the steps to manually rename the main branch (and taking care of all the
fall-out such as re-targeting existing Pull Requests).

Let's introduce a way to override Git's hard-coded default:
`core.mainBranch`.

We will start supporting this config option in the `git fmt-merge-msg`
command and successively adjust all other places where the main branch
name is hard-coded.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/config/core.txt |  5 +++++
 fmt-merge-msg.c               |  6 ++++--
 refs.c                        | 27 +++++++++++++++++++++++++++
 refs.h                        |  7 +++++++
 t/t6200-fmt-merge-msg.sh      |  7 +++++++
 5 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/core.txt b/Documentation/config/core.txt
index 74619a9c03b..32bb5368ebb 100644
--- a/Documentation/config/core.txt
+++ b/Documentation/config/core.txt
@@ -626,3 +626,8 @@ core.abbrev::
 	in your repository, which hopefully is enough for
 	abbreviated object names to stay unique for some time.
 	The minimum length is 4.
+
+core.mainBranch::
+	The name of the main (or: primary) branch in the current repository.
+	For historical reasons, `master` is used as the fall-back for this
+	setting.
diff --git a/fmt-merge-msg.c b/fmt-merge-msg.c
index 72d32bd73b1..43f4f829242 100644
--- a/fmt-merge-msg.c
+++ b/fmt-merge-msg.c
@@ -407,7 +407,7 @@ static void fmt_merge_msg_title(struct strbuf *out,
 				const char *current_branch)
 {
 	int i = 0;
-	char *sep = "";
+	char *sep = "", *main_branch;
 
 	strbuf_addstr(out, "Merge ");
 	for (i = 0; i < srcs.nr; i++) {
@@ -451,10 +451,12 @@ static void fmt_merge_msg_title(struct strbuf *out,
 			strbuf_addf(out, " of %s", srcs.items[i].string);
 	}
 
-	if (!strcmp("master", current_branch))
+	main_branch = git_main_branch_name();
+	if (!strcmp(main_branch, current_branch))
 		strbuf_addch(out, '\n');
 	else
 		strbuf_addf(out, " into %s\n", current_branch);
+	free(main_branch);
 }
 
 static void fmt_tag_signature(struct strbuf *tagbuf,
diff --git a/refs.c b/refs.c
index 224ff66c7bb..f1854cffa2f 100644
--- a/refs.c
+++ b/refs.c
@@ -560,6 +560,33 @@ void expand_ref_prefix(struct argv_array *prefixes, const char *prefix)
 		argv_array_pushf(prefixes, *p, len, prefix);
 }
 
+char *repo_main_branch_name(struct repository *r)
+{
+	const char *config_key = "core.mainbranch";
+	const char *config_display_key = "core.mainBranch";
+	const char *fall_back = "master";
+	char *name = NULL, *ret;
+
+	if (repo_config_get_string(r, config_key, &name) < 0)
+		die(_("could not retrieve `%s`"), config_display_key);
+
+	ret = name ? name : xstrdup(fall_back);
+
+	if (check_refname_format(ret, REFNAME_ALLOW_ONELEVEL))
+		die(_("invalid branch name: %s = %s"),
+		    config_display_key, name);
+
+	if (name != ret)
+		free(name);
+
+	return ret;
+}
+
+char *git_main_branch_name(void)
+{
+	return repo_main_branch_name(the_repository);
+}
+
 /*
  * *string and *len will only be substituted, and *string returned (for
  * later free()ing) if the string passed in is a magic short-hand form
diff --git a/refs.h b/refs.h
index a92d2c74c83..a207ef01348 100644
--- a/refs.h
+++ b/refs.h
@@ -154,6 +154,13 @@ int repo_dwim_log(struct repository *r, const char *str, int len, struct object_
 int dwim_ref(const char *str, int len, struct object_id *oid, char **ref);
 int dwim_log(const char *str, int len, struct object_id *oid, char **ref);
 
+/*
+ * Retrieves the name of the main (or: primary) branch of the given
+ * repository.
+ */
+char *git_main_branch_name(void);
+char *repo_main_branch_name(struct repository *r);
+
 /*
  * A ref_transaction represents a collection of reference updates that
  * should succeed or fail together.
diff --git a/t/t6200-fmt-merge-msg.sh b/t/t6200-fmt-merge-msg.sh
index e4c2a6eca43..7a873f4a05c 100755
--- a/t/t6200-fmt-merge-msg.sh
+++ b/t/t6200-fmt-merge-msg.sh
@@ -158,6 +158,13 @@ test_expect_success 'setup FETCH_HEAD' '
 	git fetch . left
 '
 
+test_expect_success 'with overridden default branch name' '
+	test_when_finished "git switch master" &&
+	git switch -c default &&
+	git -c core.mainBranch=default fmt-merge-msg <.git/FETCH_HEAD >actual &&
+	! grep "into default" actual
+'
+
 test_expect_success 'merge.log=3 limits shortlog length' '
 	cat >expected <<-EOF &&
 	Merge branch ${apos}left${apos}
-- 
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