JGit should support X.509 signed commits

[lucamilanesio]

Description

JGit should support X.509 signed commits

Motivation

Companies have official X.509 certificates and they should be able to leverage existing certified identities

Alternatives considered

JGit could leverage an external process like smimesign by spawning a process. However, that would have a detriment effect on the overall system load and latency.

Additional context

C-Git supports signed X.509 commits, therefore JGit servers (e.g. Gerrit) will have to deal with those signatures anyway.

[lucamilanesio]

@msohn by looking at BitBucket DataCentre 8.15 JGit should already support this, as BitBucket DataCentre is based on JGit. Am I mistaken?

[lucamilanesio]

@DanieleSassoli provided an initial implementation with Change eclipse-jgit/jgit/+/1193188 @msohn @tomaswolf what do you think?