[RFC/PATCH] format-patch: introduce branch.*.forkedFrom

[Ramkumar Ramachandra <artagnon@gmail.com>]

A very common workflow for preparing patches involves working off a
topic branch and generating patches against 'master' to send off to the
maintainer. However, a plain

  $ git format-patch -o outgoing

is a no-op on a topic branch, and the user has to remember to specify
'master' explicitly everytime. This problem is not unique to
format-patch; even a

  $ git rebase -i

is a no-op because the branch to rebase against isn't specified.

To tackle this problem, introduce branch.*.forkedFrom which can specify
the parent branch of a topic branch. Future patches will build
functionality around this new configuration variable.

Cc: Jeff King <peff@peff.net>
Cc: Junio C Hamano <gister@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 Since -M, -C, -D are left in the argc, checking argc < 2 isn't
 sufficient.

 I wanted to get an early reaction before wiring up checkout and
 rebase.

 But I wanted to discuss the overall idea of the patch.
 builtin/log.c           | 21 +++++++++++++++++++++
 t/t4014-format-patch.sh | 20 ++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/builtin/log.c b/builtin/log.c
index b97373d..525e696 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -674,6 +674,7 @@ static int thread;
 static int do_signoff;
 static const char *signature = git_version_string;
 static int config_cover_letter;
+static const char *config_base_branch;
 
 enum {
 	COVER_UNSET,
@@ -750,6 +751,22 @@ static int git_format_config(const char *var, const char *value, void *cb)
 		config_cover_letter = git_config_bool(var, value) ? COVER_ON : COVER_OFF;
 		return 0;
 	}
+	if (starts_with(var, "branch.")) {
+		const char *name = var + 7;
+		const char *subkey = strrchr(name, '.');
+		struct strbuf buf = STRBUF_INIT;
+
+		if (!subkey)
+			return 0;
+		strbuf_add(&buf, name, subkey - name);
+		if (branch_get(buf.buf) != branch_get(NULL))
+			return 0;
+		strbuf_release(&buf);
+		if (!strcmp(subkey, ".forkedfrom")) {
+			if (git_config_string(&config_base_branch, var, value))
+				return -1;
+		}
+	}
 
 	return git_log_config(var, value, cb);
 }
@@ -1324,6 +1341,10 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		die (_("--subject-prefix and -k are mutually exclusive."));
 	rev.preserve_subject = keep_subject;
 
+	if (argc < 2 && config_base_branch) {
+		argv[1] = config_base_branch;
+		argc++;
+	}
 	argc = setup_revisions(argc, argv, &rev, &s_r_opt);
 	if (argc > 1)
 		die (_("unrecognized argument: %s"), argv[1]);
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index 73194b2..2ea94af 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -1370,4 +1370,24 @@ test_expect_success 'cover letter auto user override' '
 	test_line_count = 2 list
 '
 
+test_expect_success 'branch.*.forkedFrom matches' '
+	mkdir -p tmp &&
+	test_when_finished "rm -rf tmp;
+		git config --unset branch.rebuild-1.forkedFrom" &&
+
+	git config branch.rebuild-1.forkedFrom master &&
+	git format-patch -o tmp >list &&
+	test_line_count = 2 list
+'
+
+test_expect_success 'branch.*.forkedFrom does not match' '
+	mkdir -p tmp &&
+	test_when_finished "rm -rf tmp;
+		git config --unset branch.foo.forkedFrom" &&
+
+	git config branch.foo.forkedFrom master &&
+	git format-patch -o tmp >list &&
+	test_line_count = 0 list
+'
+
 test_done
-- 
1.8.5.2.234.gba2dde8.dirty