Fix detection of callgrind files for very long commands
Backport fix from master A file created by a callgrind run of "ls /usr/bin/*" was not detected as callgrind file due to the previous heuristics that the first 2k characters should contain "events:". Also checking for "creator: callgrind" fixes this issue. This assumes that other creators of callgrind files never have such long command lines, are put the "event:" line before the "cmd:" line. The real fix would be a mandatory first magic line, which unfortunately is not part of the format specification, and now we need to be compatible.
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