Teach git var about GIT_EDITOR

Expose the command used by launch_editor() for scripts to use.
This should allow one to avoid searching for a proper editor
separately in each command.

git_editor(void) uses the logic to decide which editor to use
that used to live in launch_editor().  The function returns NULL
if there is no suitable editor; the caller is expected to issue
an error message when appropriate.

launch_editor() uses git_editor() and gives the error message the
same way as before when EDITOR is not set.

"git var GIT_EDITOR" gives the editor name, or an error message
when there is no appropriate one.

"git var -l" gives GIT_EDITOR=name only if there is an
appropriate editor.

Originally-submitted-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

@@ -36,6 +36,14 @@ GIT_AUTHOR_IDENT::
 GIT_COMMITTER_IDENT::
     The person who put a piece of code into git.
 
+GIT_EDITOR::
+    Text editor for use by git commands.  The value is meant to be
+    interpreted by the shell when it is used.  Examples: `~/bin/vi`,
+    `$SOME_ENVIRONMENT_VARIABLE`, `"C:\Program Files\Vim\gvim.exe"
+    --nofork`.  The order of preference is the `$GIT_EDITOR`
+    environment variable, then `core.editor` configuration, then
+    `$VISUAL`, then `$EDITOR`, and then finally 'vi'.
+
 Diagnostics
 -----------
 You don't exist. Go away!::

cache.h

@@ -750,6 +750,7 @@ extern const char *git_author_info(int);
 extern const char *git_committer_info(int);
 extern const char *fmt_ident(const char *name, const char *email, const char *date_str, int);
 extern const char *fmt_name(const char *name, const char *email);
+extern const char *git_editor(void);
 
 struct checkout {
 	const char *base_dir;

editor.c

@@ -2,7 +2,7 @@
 #include "strbuf.h"
 #include "run-command.h"
 
-int launch_editor(const char *path, struct strbuf *buffer, const char *const *env)
+const char *git_editor(void)
 {
 	const char *editor = getenv("GIT_EDITOR");
 	const char *terminal = getenv("TERM");
@@ -16,11 +16,21 @@ int launch_editor(const char *path, struct strbuf *buffer, const char *const *en
 		editor = getenv("EDITOR");
 
 	if (!editor && terminal_is_dumb)
-		return error("terminal is dumb, but EDITOR unset");
+		return NULL;
 
 	if (!editor)
 		editor = "vi";
 
+	return editor;
+}
+
+int launch_editor(const char *path, struct strbuf *buffer, const char *const *env)
+{
+	const char *editor = git_editor();
+
+	if (!editor)
+		return error("Terminal is dumb, but EDITOR unset");
+
 	if (strcmp(editor, ":")) {
 		size_t len = strlen(editor);
 		int i = 0;

var.c

@@ -8,21 +8,35 @@
 
 static const char var_usage[] = "git var [-l | <variable>]";
 
+static const char *editor(int flag)
+{
+	const char *pgm = git_editor();
+
+	if (!pgm && flag & IDENT_ERROR_ON_NO_NAME)
+		die("Terminal is dumb, but EDITOR unset");
+
+	return pgm;
+}
+
 struct git_var {
 	const char *name;
 	const char *(*read)(int);
 };
 static struct git_var git_vars[] = {
 	{ "GIT_COMMITTER_IDENT", git_committer_info },
 	{ "GIT_AUTHOR_IDENT",   git_author_info },
+	{ "GIT_EDITOR", editor },
 	{ "", NULL },
 };
 
 static void list_vars(void)
 {
 	struct git_var *ptr;
+	const char *val;
+
 	for (ptr = git_vars; ptr->read; ptr++)
-		printf("%s=%s\n", ptr->name, ptr->read(0));
+		if ((val = ptr->read(0)))
+			printf("%s=%s\n", ptr->name, val);
 }
 
 static const char *read_var(const char *var)