Suggest reviewers fails if HEAD is ambiguous

[Adam Ahmed]

Suggest reviewers is using the default CommandExitHandler. That means "warning" lines on stderr are treated as fatal, resulting in:

2015-04-09 19:13:29,603 ERROR [http-nio-9080-exec-37] mklinski *7L2QXZx1153x3465316x1 nwk4rt 172.28.13.91,172.24.12.146 "GET /rest/suggest-reviewers/1.0/by/ref HTTP/1.0" c.a.s.s.i.ScoringSuggestedReviewerService class com.atlassian.stash.suggestreviewers.internal.suggester.BlameSuggester threw an exception (or returned unexpected data) when suggesting reviewers and was ignored.
com.atlassian.stash.exception.CommandFailedException: '/usr/bin/git blame --incremental 72a317eca420ace9117768e474472d5dd395247d -- src/objects/Opportunity.object' exited with code 0 saying: warning: refname 'HEAD' is ambiguous.
        at com.atlassian.stash.scm.DefaultCommandExitHandler.onError(DefaultCommandExitHandler.java:47) ~[stash-spi-3.8.0.jar:na]
        at com.atlassian.stash.suggestreviewers.internal.suggester.git.MissingPathIgnoringExitHandler.onError(MissingPathIgnoringExitHandler.java:28) ~[plugin.3653444971601458049.stash-suggest-reviewers-1.4.jar:na]

Since the exit code was 0, that command shouldn't have failed.

[Bryan Turner]

Duplicate of #19.