Prevent double-login for Git HTTP and LFS and simplify login (#15303)

* Prevent double-login for Git HTTP and LFS and simplify login

There are a number of inconsistencies with our current methods for
logging in for git and lfs. The first is that there is a double login
process. This is particularly evident in 1.13 where there are no less
than 4 hash checks for basic authentication due to the previous
IsPasswordSet behaviour.

This duplicated code had individual inconsistencies that were not
helpful and caused confusion.

This PR does the following:

* Remove the specific login code from the git and lfs handlers except
for the lfs special bearer token
* Simplify the meaning of DisableBasicAuthentication to allow Token and
Oauth2 sign-in.
* The removal of the specific code from git and lfs means that these
both now have the same login semantics and can - if not
DisableBasicAuthentication - login from external services. Further it
allows Oauth2 token authentication as per our standard mechanisms.
* The change in the recovery handler prevents the service from
re-attempting to login - primarily because this could easily cause a
further panic and it is wasteful.

* add test

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Andrew Thornton <art27@cantab.net>

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