remote rename: more carefully determine whether a remote is configured

remote rename: more carefully determine whether a remote is configured

One of the really nice features of the ~/.gitconfig file is that users
can override defaults by their own preferred settings for all of their
repositories.

One such default that some users like to override is whether the
"origin" remote gets auto-pruned or not. The user would simply call

	git config --global remote.origin.prune true

and from now on all "origin" remotes would be pruned automatically when
fetching into the local repository.

There is just one catch: now Git thinks that the "origin" remote is
configured, even if the repository config has no [remote "origin"]
section at all, as it does not realize that the "prune" setting was
configured globally and that there really is no "origin" remote
configured in this repository.

That is a problem e.g. when renaming a remote to a new name, when Git
may be fooled into thinking that there is already a remote of that new
name.

Let's fix this by paying more attention to *where* the remote settings
came from: if they are configured in the local repository config, we
must not overwrite them. If they were configured elsewhere, we cannot
overwrite them to begin with, as we only write the repository config.

There is only one caller of remote_is_configured() (in `git fetch`) that
may want to take remotes into account even if they were configured
outside the repository config; all other callers essentially try to
prevent the Git command from overwriting settings in the repository
config.

To accommodate that fact, the remote_is_configured() function now
requires a parameter that states whether the caller is interested in all
remotes, or only in those that were configured in the repository config.

Many thanks to Jeff King whose tireless review helped with settling for
nothing less than the current strategy.

This fixes https://github.com/git-for-windows/git/issues/888

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>