Automatically link commit ranges to compare page.

Closes #2103

Implemented as proposed in the last paragraph of the issue:

> We could do something similar to Ruby, where `1..5` means `1,2,3,4,5` and `1...5` means `1,2,3,4`: `..` means inclusive, `...` means exclusive. In our case, `sha1...sha4` would mean `sha2,sha3,sha4` (exclusive with regards to `sha1`) and `sha1..sha4` would mean `sha1^...sha4`, i.e. `sha1,sha2,sha3,sha4` (inclusive to `sha1`).

- `sha1...sha4` now links to `compare/sha1...sha4`
- `sha1..sha4` now links to `compare/sha1^...sha4`.

See merge request !1649