format-patch subject-prefix gets truncated when using the --numbered flag

Hello,

I noticed that the --subject-prefix string gets truncated sometimes, but only when using the --numbered flat. Here's an example:

addu@sestofb11:/data/fb/addu/git$ export longm="very very very very very very very very very very very very very very long prefix"

addu@sestofb11:/data/fb/addu/git$ git format-patch -1 --subject- prefix="$longm][PATCH"

As expected, in the generated patch file we have:
Subject: [very very very very very very very very very very very very
very very long prefix][PATCH] 
 First batch after 2.12

But now, if I pass the --numbered flag too: addu@sestofb11:/data/fb/addu/git$ git format-patch -1 --numbered -- subject-prefix="$longm][PATCH"

In the generated patch file we get this: 
Subject: [very very very very very very very very very very verFirst
batch 
 after 2.12

This is happening on the latest master branch, so I dug through the code and tracked the issue to this piece of code in log-tree.c:

        if (opt->total > 0) {
                static char buffer[64];
                snprintf(buffer, sizeof(buffer),
                         "Subject: [%s%s%0*d/%d] ",
                         opt->subject_prefix,
                         *opt->subject_prefix ? " " : "",
                         digits_in_number(opt->total),
                         opt->nr, opt->total);
                subject = buffer;
        } else if (opt->total == 0 && opt->subject_prefix && *opt-

                static char buffer[256];
                snprintf(buffer, sizeof(buffer),
                         "Subject: [%s] ",
                         opt->subject_prefix);
                subject = buffer;
        } else {
                subject = "Subject: ";
        }

Apparently the size of the "buffer" var is different in the two situations. Anybody knows if this is by design or just an old oversight? I can send a patch to fix it but I'm not very familiar with the git code and I'm afraid some hidden consequence I don't see right now.

Cheers, --Adrian