launch_editor(): Heed GIT_EDITOR and core.editor settings

In the commit 'Add GIT_EDITOR environment and core.editor
configuration variables', this was done for the shell scripts.
Port it over to builtin-tag's version of launch_editor(), which
is just about to be refactored into editor.c.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -24,7 +24,11 @@ static void launch_editor(const char *path, char **buffer, unsigned long *len)
 	const char *args[3];
 	int fd;
 
-	editor = getenv("VISUAL");
+	editor = getenv("GIT_EDITOR");
+	if (!editor && editor_program)
+		editor = editor_program;
+	if (!editor)
+		editor = getenv("VISUAL");
 	if (!editor)
 		editor = getenv("EDITOR");
 
@@ -249,9 +253,9 @@ static void create_tag(const unsigned char *object, const char *tag,
 		       char *message, int sign, unsigned char *result)
 {
 	enum object_type type;
-	char header_buf[1024], *buffer;
+	char header_buf[1024], *buffer = NULL;
 	int header_len, max_size;
-	unsigned long size;
+	unsigned long size = 0;
 
 	type = sha1_object_info(object, NULL);
 	if (type <= 0)

cache.h

@@ -560,6 +560,8 @@ extern char *pager_program;
 extern int pager_in_use;
 extern int pager_use_color;
 
+extern char *editor_program;
+
 /* base85 */
 int decode_85(char *dst, const char *line, int linelen);
 void encode_85(char *buf, const unsigned char *data, int bytes);

config.c

@@ -426,6 +426,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.editor")) {
+		editor_program = xstrdup(value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }

environment.c

@@ -33,6 +33,7 @@ size_t delta_base_cache_limit = 16 * 1024 * 1024;
 char *pager_program;
 int pager_in_use;
 int pager_use_color = 1;
+char *editor_program;
 int auto_crlf = 0;	/* 1: both ways, -1: only when adding git objects */
 
 static const char *git_dir;