* bf/set-head-symref:
  fetch set_head: handle mirrored bare repositories
  fetch: set remote/HEAD if it does not exist
  refs: add create_only option to refs_update_symref_extended
  refs: add TRANSACTION_CREATE_EXISTS error
  remote set-head: better output for --auto
  remote set-head: refactor for readability
  refs: atomically record overwritten ref in update_symref
  refs: standardize output of refs_read_symbolic_ref
  t/t5505-remote: test failure of set-head
  t/t5505-remote: set default branch to main

In the current implementation, if refs/remotes/$remote/HEAD does not
exist, running fetch will create it, but if it does exist it will not do
anything, which is a somewhat safe and minimal approach. Unfortunately,
for users who wish to NOT have refs/remotes/$remote/HEAD set for any
reason (e.g. so that `git rev-parse origin` doesn't accidentally point
them somewhere they do not want to), there is no way to remove this
behaviour. On the other side of the spectrum, users may want fetch to
automatically update HEAD or at least give them a warning if something
changed on the remote.

Introduce a new setting, remote.$remote.followRemoteHEAD with four
options:

    - "never": do not ever do anything, not even create
    - "create": the current behaviour, now the default behaviour
    - "warn": print a message if remote and local HEAD is different
    - "always": silently update HEAD on every change

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Advice about what to do when getting a warning is typed out explicitly
twice and is printed as regular output. The output is also tested for.
Extract the advice message into a single place and use a wrapper
function, so if later the advice is made more chatty the signature only
needs to be changed in once place. Remove the testing for the advice
output in the tests.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Currently if we want to have a remote/HEAD locally that is different
from the one on the remote, but we still want to get a warning if remote
changes HEAD, our only option is to have an indiscriminate warning with
"follow_remote_head" set to "warn". Add a new option
"warn-if-not-$branch", where $branch is a branch name we do not wish to
get a warning about. If the remote HEAD is $branch do not warn,
otherwise, behave as "warn".

E.g. let's assume, that our remote origin has HEAD
set to "master", but locally we have "git remote set-head origin seen".
Setting 'remote.origin.followRemoteHEAD = "warn"' will always print
a warning, even though the remote has not changed HEAD from "master".
Setting 'remote.origin.followRemoteHEAD = "warn-if-not-master" will
squelch the warning message, unless the remote changes HEAD from
"master". Note, that should the remote change HEAD to "seen" (which we
have locally), there will still be no warning.

Improve the advice message in report_set_head to also include silencing
the warning message with "warn-if-not-$branch".

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

When running "remote set-head" manually it is unlikely, that the user
would actually like to have "fetch" always update the remote/HEAD. On
the contrary, it is more likely, that the user would expect remote/HEAD
to stay the way they manually set it, and just forgot about having
"followRemoteHEAD" set to "always".

When "followRemoteHEAD" is set to "always" make running "remote
set-head" change the config to "warn".

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

"git fetch" honors "remote.<remote>.followRemoteHEAD" settings to
tweak the remote-tracking HEAD in "refs/remotes/<remote>/HEAD".

* bf/fetch-set-head-config:
  remote set-head: set followRemoteHEAD to "warn" if "always"
  fetch set_head: add warn-if-not-$branch option
  fetch set_head: move warn advice into advise_if_enabled
  fetch: add configuration for set_head behaviour