Allow custom "comment char"

Some users do want to write a line that begin with a pound sign, #,
in their commit log message.  Many tracking system recognise
a token of #<bugid> form, for example.

The support we offer these use cases is not very friendly to the end
users.  They have a choice between

 - Don't do it.  Avoid such a line by rewrapping or indenting; and

 - Use --cleanup=whitespace but remove all the hint lines we add.

Give them a way to set a custom comment char, e.g.

    $ git -c core.commentchar="%" commit

so that they do not have to do either of the two workarounds.

[jc: although I started the topic, all the tests and documentation
updates, many of the call sites of the new strbuf_add_commented_*()
functions, and the change to git-submodule.sh scripted Porcelain are
from Ralf.]

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -706,11 +706,11 @@ static int edit_branch_description(const char *branch_name)
 	read_branch_desc(&buf, branch_name);
 	if (!buf.len || buf.buf[buf.len-1] != '\n')
 		strbuf_addch(&buf, '\n');
-	strbuf_addf(&buf,
-		    "# Please edit the description for the branch\n"
-		    "#   %s\n"
-		    "# Lines starting with '#' will be stripped.\n",
-		    branch_name);
+	strbuf_commented_addf(&buf,
+		    "Please edit the description for the branch\n"
+		    "  %s\n"
+		    "Lines starting with '%c' will be stripped.\n",
+		    branch_name, comment_line_char);
 	fp = fopen(git_path(edit_description), "w");
 	if ((fwrite(buf.buf, 1, buf.len, fp) < buf.len) || fclose(fp)) {
 		strbuf_release(&buf);