multimedia/mpv: unbreak fetch via local copy

Index abbreviation in git-format-patch(1) output seems to change
frequently, probably due to GitHub load balancing.

$ make checksum TRYBROKEN= MASTER_SITE_BACKUP= DISTDIR=$(mktemp -dt distfiles)
[...]
=> 62fb374349e0.patch doesn't seem to exist in /tmp/distfiles.kipot6aT/.
=> Attempting to fetch https://github.com/mpv-player/mpv/commit/62fb374349e0.patch
fetch: https://github.com/mpv-player/mpv/commit/62fb374349e0.patch: size mismatch: expected 2083, actual 2079
=> Couldn't fetch it - please try to retrieve this
=> port manually into /tmp/distfiles.kipot6aT/ and try again.
*** Error code 1

Reported by:	LuMiWa (via multimedia@ list)