Support specifying a checksum for github repos.
Support specifying a checksum for github repos.

Bazel can cache downloads better if a checksum is specified.

pmuetschard/gapid,pmuetschard/gapid,pmuetschard/gapid,google/gapid,google/gapid,google/gapid,google/agi,pmuetschard/gapid,google/gapid,pmuetschard/gapid,google/agi,google/agi,google/agi,pmuetschard/gapid,google/gapid,pmuetschard/gapid,google/agi,google/agi,google/gapid,google/gapid,pmuetschard/gapid,google/agi,google/gapid,google/agi