I would not say the patch is wrong per-se, but:
- People who do not override prefix to install in $HOME would
suddenly get their mandir under $HOME/share/man; the fact
nobody complained so far suggests me that these people have
$MANPATH pointing at $HOME/man, which means your patch breaks
things for them, unless they have been overriding mandir in
which case your patch does not matter to them;
- Distros who package git and want to conform to FHS would have
been overriding not just prefix but mandir anyway, so your patch
is not an improvement to them;
- People who override prefix to install locally in /usr or
/usr/local does "make prefix=" thing anyway, and the fact
nobody complained so far suggests me that they have already
known they need to override mandir as well if they do not
like /usr/local/man, so your patch is not an improvement to
them.
That makes one half negative plus two and half neutral.