Solves #2602 - the new version of alertmanager introduces an isEqual
flag to support != matchers in alertmanager silences. Clients set this
to true in the JSON blob sent to the api to indicate a foo=bar matcher,
and to false to indicate foo!=bar. Due to gos unmarshalling, anything
that _doesn't_ include this flag will default to false (i.e. a !=
matcher), so any clients that aren't aware of this flag (such as amtools
before negative matchers and the new api) will not send it, and when you
think you are making a foo=bar matcher, alertmanager will interpret that
as a not equals.
This commit changes the Unmarshaling of the v1matcher struct to default
the IsEqual flag to true, to keep the old behaviour for clients not
setting the flag
Signed-off-by: sinkingpoint <colin@quirl.co.nz>