dashboard: record all git revisions on which a bug was seen

[a-nogikh]

Discussed here: https://groups.google.com/g/syzkaller/c/UTPrWcJfS8Q/m/a2HU4X2SAAAJ

If, for each bug, we store and display the full list of kernel revisions where it was detected, it may help to better diagnose the problem, especially for heisenbugs.

Currently, we do show some kernel revisions in the crashes table, but this list is limited in size and we eventually start skipping significant portions of the history.

Displaying the revisions on which the bug was not detected likely does not make much sense in our case, unless we start re-testing all reproducers regularly.