rev-list: add --allow-missing-tips to be used with --missing=...

In 9830926c (rev-list: add commit object support in --missing option, 2023-10-27) we fixed the --missing option in git rev-list so that it now works with commits too.

Unfortunately, such a command would still fail with a "fatal: bad object " if it is passed a missing commit, blob or tree as an argument.

When such a command is used to find the dependencies of some objects, for example the dependencies of quarantined objects, it would be better if the command would instead consider such missing objects, especially commits, in the same way as other missing objects.

If, for example --missing=print is used, it would be nice for some use cases if the missing tips passed as arguments were reported in the same way as other missing objects instead of the command just failing.

Let's introduce a new --allow-missing-tips option to make it work like this.

Signed-off-by: Christian Couder chriscool@tuxfamily.org

Related to #234 (closed)