launch_editor(): allow spaces in the filename

The construct

	sh -c "$0 \"$@\"" <editor> <file>

does not pick up quotes in <editor>, so you cannot give path to the
editor that has a shell IFS whitespace in it, and also give it initial
set of parameters and flags.  Replace $0 with <editor> to fix this issue.

This fixes

	git config core.editor '"c:/Program Files/What/Ever.exe"'

In other words, you can specify an editor with spaces in its path using a
config containing something like this:

	[core]
		editor = \"c:/Program Files/Darn/Spaces.exe\"

NOTE: we cannot just replace the $0 with \"$0\", because we still want
this to work:

	[core]
		editor = emacs -nw

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

builtin-tag.c

@@ -50,19 +50,23 @@ void launch_editor(const char *path, struct strbuf *buffer, const char *const *e
 		size_t len = strlen(editor);
 		int i = 0;
 		const char *args[6];
+		struct strbuf arg0;
 
+		strbuf_init(&arg0, 0);
 		if (strcspn(editor, "$ \t'") != len) {
 			/* there are specials */
+			strbuf_addf(&arg0, "%s \"$@\"", editor);
 			args[i++] = "sh";
 			args[i++] = "-c";
-			args[i++] = "$0 \"$@\"";
+			args[i++] = arg0.buf;
 		}
 		args[i++] = editor;
 		args[i++] = path;
 		args[i] = NULL;
 
 		if (run_command_v_opt_cd_env(args, 0, NULL, env))
 			die("There was a problem with the editor %s.", editor);
+		strbuf_release(&arg0);
 	}
 
 	if (!buffer)

t/t7005-editor.sh

@@ -89,6 +89,33 @@ do
 	'
 done
 
+test_expect_success 'editor with a space' '
+
+	if echo "echo space > \"\$1\"" > "e space.sh"
+	then
+		chmod a+x "e space.sh" &&
+		GIT_EDITOR="./e\ space.sh" git commit --amend &&
+		test space = "$(git show -s --pretty=format:%s)"
+	else
+		say "Skipping; FS does not support spaces in filenames"
+	fi
+
+'
+
+unset GIT_EDITOR
+test_expect_success 'core.editor with a space' '
+
+	if test -f "e space.sh"
+	then
+		git config core.editor \"./e\ space.sh\" &&
+		git commit --amend &&
+		test space = "$(git show -s --pretty=format:%s)"
+	else
+		say "Skipping; FS does not support spaces in filenames"
+	fi
+
+'
+
 TERM="$OLD_TERM"
 
 test_done