var: add GIT_SEQUENCE_EDITOR variable

The editor program used by Git when editing the sequencer "todo" file
is determined by examining a few environment variables and also
affected by configuration variables. Introduce "git var
GIT_SEQUENCE_EDITOR" to give users access to the final result of the
logic without having to know the exact details.

This is very similar in spirit to 44fcb497 (Teach git var about
GIT_EDITOR, 2009-11-11) that introduced "git var GIT_EDITOR".

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

diff --git a/Documentation/git-var.txt b/Documentation/git-var.txt
index 0ab5bfa..f40202b 100644
--- a/Documentation/git-var.txt
+++ b/Documentation/git-var.txt
@@ -50,6 +50,14 @@
     The build you are using chose '{git-default-editor}' as the default.
 endif::git-default-editor[]
 
+GIT_SEQUENCE_EDITOR::
+    Text editor used to edit the 'todo' file while running `git rebase
+    -i`. Like `GIT_EDITOR`, the value is meant to be interpreted by
+    the shell when it is used. The order of preference is the
+    `$GIT_SEQUENCE_EDITOR` environment variable, then
+    `sequence.editor` configuration, and then the value of `git var
+    GIT_EDITOR`.
+
 GIT_PAGER::
     Text viewer for use by Git commands (e.g., 'less').  The value
     is meant to be interpreted by the shell.  The order of preference

builtin/var.c

diff --git a/builtin/var.c b/builtin/var.c
index a1a2522..a80c1df 100644
--- a/builtin/var.c
+++ b/builtin/var.c
@@ -14,6 +14,11 @@ static const char *editor(int flag)
 	return git_editor();
 }
 
+static const char *sequence_editor(int flag)
+{
+	return git_sequence_editor();
+}
+
 static const char *pager(int flag)
 {
 	const char *pgm = git_pager(1);
@@ -36,6 +41,7 @@ static struct git_var git_vars[] = {
 	{ "GIT_COMMITTER_IDENT", git_committer_info },
 	{ "GIT_AUTHOR_IDENT",   git_author_info },
 	{ "GIT_EDITOR", editor },
+	{ "GIT_SEQUENCE_EDITOR", sequence_editor },
 	{ "GIT_PAGER", pager },
 	{ "GIT_DEFAULT_BRANCH", default_branch },
 	{ "", NULL },

t/t0007-git-var.sh

diff --git a/t/t0007-git-var.sh b/t/t0007-git-var.sh
index 433d242..eeb8539 100755
--- a/t/t0007-git-var.sh
+++ b/t/t0007-git-var.sh
@@ -109,6 +109,44 @@
 	)
 '
 
+test_expect_success 'get GIT_SEQUENCE_EDITOR without configuration' '
+	(
+		sane_unset GIT_SEQUENCE_EDITOR &&
+		git var GIT_EDITOR >expect &&
+		git var GIT_SEQUENCE_EDITOR >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'get GIT_SEQUENCE_EDITOR with configuration' '
+	test_config sequence.editor foo &&
+	(
+		sane_unset GIT_SEQUENCE_EDITOR &&
+		echo foo >expect &&
+		git var GIT_SEQUENCE_EDITOR >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'get GIT_SEQUENCE_EDITOR with environment variable' '
+	(
+		sane_unset GIT_SEQUENCE_EDITOR &&
+		echo bar >expect &&
+		GIT_SEQUENCE_EDITOR=bar git var GIT_SEQUENCE_EDITOR >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'get GIT_SEQUENCE_EDITOR with configuration and environment variable' '
+	test_config sequence.editor foo &&
+	(
+		sane_unset GIT_SEQUENCE_EDITOR &&
+		echo bar >expect &&
+		GIT_SEQUENCE_EDITOR=bar git var GIT_SEQUENCE_EDITOR >actual &&
+		test_cmp expect actual
+	)
+'
+
 # For git var -l, we check only a representative variable;
 # testing the whole output would make our test too brittle with
 # respect to unrelated changes in the test suite's environment.