Add gitref macro to reference a git hash

With this extension a git hash can be easily referenced.

The porters handbook versions chapter uses this extension
as an example in the FreeBSD 14.0 version table.

An example of use

gitref:hash[repository="src|doc|ports",length=hash-longitude]

The repository should be src, doc or ports.
And if no length it's specified the macro will use 12 characters
of the specified hash.

PR: 253050
Submitted by: jhb@