fetch: refactor do_fetch handling of followRemoteHEAD

fetch: refactor do_fetch handling of followRemoteHEAD

Update enum follow_remote_head_settings to include the value
FOLLOW_REMOTE_UNCONFIGURED as the new zero-initialized value for
followRemoteHEAD.  This will allow us to distinguish between the
variable being unset vs. explicitly set to 'create', which is ultimately
the system default.  The unnecessary indentation is removed.

The do_fetch function is likewise updated to perform its own decision
making to determine the effective followRemoteHEAD mode, falling back to
the system default if necessary.  This will enable the next patch to
introduce a user-configurable default.

Function set_head now accepts the mode as an argument rather than only
considering the value defined by the remote.

The use of the 'warn-if-not-$branch' value is awkward in the context of
a global default, since the branches will differ between individual
remotes.  For this reason, it's left out of this scheme and handling of
the no_warn_branch variable is untouched.  Since a remote-specific
value for followRemoteHEAD takes priority, we can assume that if
remote->no_warn_branch is set, then the remote is also asserting
FOLLOW_REMOTE_WARN as the effective operating mode, and it will be
honored by do_fetch.

Signed-off-by: Matt Hunter <redacted>
Signed-off-by: Junio C Hamano <redacted>