The list of commits on the New Review Request page was blank for repositories

using rb-gateway. This was due to Time.String() mishandling the timezone

offset. The problematic instances of Time.String() were replaced by

Time.Format("2006-01-02T15:04:05-0700").

Initially, requesting a list of commits yielded entries like the one below.

Notice the repetition of the timezone offset -0400 -0400.
http :8888/repos/testRepo1/branches/master/commits
PRIVATE-TOKEN:dXNlcm5hbWU6cGFzc3dvcmQ=
"author": "Henri-Philippe Marceau", 
"date": "2017-10-08 12:17:21 -0400 -0400", 
"id": "704456ac9ec646c4f6316f8975d3d04fab32d7cb", 
"message": "modified FILEA\n", 
"parent_id": "2dd9a09b624597179d19b05da5b503bc4249f3eb"

Then, after the first attempt at fixing the problem, the timezone offset

occurred only once but lost its leading zero and was incorrect -700.
http :8888/repos/testRepo1/branches/master/commits
PRIVATE-TOKEN:dXNlcm5hbWU6cGFzc3dvcmQ=
"author": "Henri-Philippe Marceau", 
"date": "2017-10-08T12:17:21-700", 
"id": "704456ac9ec646c4f6316f8975d3d04fab32d7cb", 
"message": "modified FILEA\n", 
"parent_id": "2dd9a09b624597179d19b05da5b503bc4249f3eb"

Finally, Time.Format("2006-01-02T15:04:05-0700") was introduced, and the

desired date-time output was achieved, -0400.
http :8888/repos/testRepo1/branches/master/commits
PRIVATE-TOKEN:dXNlcm5hbWU6cGFzc3dvcmQ=
"author": "Henri-Philippe Marceau", 
"date": "2017-10-08T12:17:21-0400", 
"id": "704456ac9ec646c4f6316f8975d3d04fab32d7cb", 
"message": "modified FILEA\n", 
"parent_id": "2dd9a09b624597179d19b05da5b503bc4249f3eb"

To maintain consistency, both GetCommits & GetCommit, in git_repository.go,

were updated. The following is the output of a GetCommit request.
http :8888/repos/testRepo1/commits/8619c0d73748feeaa568c3c2a5378ad5332efa84
PRIVATE-TOKEN:dXNlcm5hbWU6cGFzc3dvcmQ=
"author": "Henri-Philippe Marceau", 
"date": "2017-09-29T21:52:51-0400",
...

To ensure that these changes didn't break anything, the unit tests were run

with go test:
PASS
ok      github.com/reviewboard/rb-gateway   0.096s

Also, the lists of commits for repositories using rb-gateway are now being

displayed.