editor: add a function to launch the sequence editor

As part of the rewrite of interactive rebase, the sequencer will need to
open the sequence editor to allow the user to edit the todo list.
Instead of duplicating the existing launch_editor() function, this
refactors it to a new function, launch_specified_editor(), which takes
the editor as a parameter, in addition to the path, the buffer and the
environment variables.  launch_sequence_editor() is then added to launch
the sequence editor.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1409,6 +1409,7 @@ extern const char *fmt_name(const char *name, const char *email);
 extern const char *ident_default_name(void);
 extern const char *ident_default_email(void);
 extern const char *git_editor(void);
+extern const char *git_sequence_editor(void);
 extern const char *git_pager(int stdout_is_tty);
 extern int is_terminal_dumb(void);
 extern int git_ident_config(const char *, const char *, void *);

editor.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "config.h"
 #include "strbuf.h"
 #include "run-command.h"
 #include "sigchain.h"
@@ -34,10 +35,21 @@ const char *git_editor(void)
 	return editor;
 }
 
-int launch_editor(const char *path, struct strbuf *buffer, const char *const *env)
+const char *git_sequence_editor(void)
 {
-	const char *editor = git_editor();
+	const char *editor = getenv("GIT_SEQUENCE_EDITOR");
+
+	if (!editor)
+		git_config_get_string_const("sequence.editor", &editor);
+	if (!editor)
+		editor = git_editor();
 
+	return editor;
+}
+
+static int launch_specified_editor(const char *editor, const char *path,
+				   struct strbuf *buffer, const char *const *env)
+{
 	if (!editor)
 		return error("Terminal is dumb, but EDITOR unset");
 
@@ -95,3 +107,14 @@ int launch_editor(const char *path, struct strbuf *buffer, const char *const *en
 		return error_errno("could not read file '%s'", path);
 	return 0;
 }
+
+int launch_editor(const char *path, struct strbuf *buffer, const char *const *env)
+{
+	return launch_specified_editor(git_editor(), path, buffer, env);
+}
+
+int launch_sequence_editor(const char *path, struct strbuf *buffer,
+			   const char *const *env)
+{
+	return launch_specified_editor(git_sequence_editor(), path, buffer, env);
+}

strbuf.h

@@ -575,6 +575,8 @@ extern void strbuf_add_unique_abbrev(struct strbuf *sb,
  * file's contents are not read into the buffer upon completion.
  */
 extern int launch_editor(const char *path, struct strbuf *buffer, const char *const *env);
+extern int launch_sequence_editor(const char *path, struct strbuf *buffer,
+				  const char *const *env);
 
 extern void strbuf_add_lines(struct strbuf *sb, const char *prefix, const char *buf, size_t size);