strbuf_readlink(): avoid calling `readlink()` twice in corner-cases

strbuf_readlink(): avoid calling `readlink()` twice in corner-cases

The `strbuf_readlink()` function calls `readlink()`` twice if the hint
argument specifies the exact size of the link target (e.g. by passing
stat.st_size as returned by `lstat()`). This is necessary because
`readlink(..., hint) == hint` could mean that the buffer was too small.

Use `hint + 1` as buffer size to prevent this.

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