Add dereference option to gert::git_tag_list

[jabenninghoff]

gert::git_tag_list() returns the SHA1 hash of the tag, like the command git show-ref --tags.

It would be helpful to optionally return the SHA1 hash of the commit for the tag, like git show-ref --tags --dereference, which would (for example) allow joining with the results of gert::git_log().