get_git_head_revision in detached mode

[alinagrigoras]

Hi,

The tag is not correctly propagated if the tag is in a detached mode.
This is basically the same issue as the issue below, but instead of a branch the information needed is a tag.
#3

You can recreate the issue by creating a tag on a different branch than master, for example, the branch should not be checkout, but you directly check out the tag.

Cheers
Alina

[rpavlik]

I think we can simplify the code here somewhat - some of it was to avoid having to shell out to Git, and some of it was to latch on to particular files to trigger a re-configure if important things happened in Git.