pull: improve advice for unconfigured error case

pull: improve advice for unconfigured error case

There are several reasons a git-pull invocation might not
have anything marked for merge:

  1. We're not on a branch, so there is no branch
     configuration.

  2. We're on a branch, but there is no configuration for
     this branch.

  3. We fetched from the configured remote, but the
     configured branch to merge didn't get fetched (either
     it doesn't exist, or wasn't part of the fetch refspec).

  4. We fetched from the non-default remote, but didn't
     specify a branch to merge. We can't use the configured
     one because it applies to the default remote.

  5. We fetched from a specified remote, and a refspec was
     given, but it ended up not fetching anything (this is
     actually hard to do; if the refspec points to a remote
     branch and it doesn't exist, then fetch will fail and
     we never make it to this code path. But if you provide
     a wildcard refspec like

       refs/bogus/*:refs/remotes/origin/*

     then you can see this failure).

We have handled (1) and (2) for some time. Recently, commit
a6dbf88 added code to handle case (3).

This patch handles cases (4) and (5), which previously just
fell under other cases, producing a confusing message.

While we're at it, let's rewrap the text for case (3), which
looks terribly ugly as it is.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>