Missing support for short-hash archives download

[strk]

While short-hash are recognised by the commit browser, for example:

• https://try.gogs.io/strk/trac2gogs/commit/addbbc52

The same short reference doesn't work at fetching a tarball:

• https://try.gogs.io/strk/trac2gogs/archive/addbbc52.tar.gz

While full hash does work:

• https://try.gogs.io/strk/trac2gogs/archive/addbbc524c52f271c2542009bc947c8200b588a9.tar.gz

It would be nice if hashes recognition was consistent.

See how the first one was fixed in gogs/gogs#3321

Ticket migrated from gogs/gogs#3834