am: fix msgnum_sb pointer as sb use strbuf

[GermanAizek]

@metux,
This change protect pointer to a static buffer that can be overwritten by a subsequent mkpath call. Previous developers applied this to sb, which is a good practice, but forgot for msgnum_sb.

References:

• https://www.spinics.net/lists/git/msg239806.html

This patch makes
get_pathname() return a strbuf and updates the callers to take
advantage of this. The code is simplified as we no longer need to
worry about buffer overflow.