diagnose_invalid_index_path: use strbuf to avoid strcpy/strcat

diagnose_invalid_index_path: use strbuf to avoid strcpy/strcat

We dynamically allocate a buffer and then strcpy and strcat
into it. This isn't buggy, but we'd prefer to avoid these
suspicious functions.

This would be a good candidate for converstion to xstrfmt,
but we need to record the length for dealing with index
entries. A strbuf handles that for us.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>