Include recent command history in fast-import crash reports
When we crash the frontend developer (or end-user) may need to know roughly around what part of the input stream we had a problem with and aborted on. Because line numbers aren't very useful in this sort of application we instead just keep the last 100 commands in a FIFO queue and print them as part of the crash report.
Currently one problem with this design is a commit that has more than 100 modified files in it will flood the FIFO and any context regarding branch/from/committer/mark/comments will be lost. We really should save only the last few (10?) file changes for the current commit, ensuring we have some prior higher level commands in the FIFO when we crash on a file M/D/C/R command.
Another issue with this approach is the FIFO only includes the commands, it does not include the commit messages. Yet having a commit message may be useful to help locate the relevant change in the source material. In practice I don't think this is going to be a major concern as the frontend can always embed its own source change set identifier as a comment (which will appear in the crash report) and the commit message(s) for the most recent commits of any given branch should be obtainable from the (packed) commit objects.
Signed-off-by: Shawn O. Pearce <email@example.com>