branch --track: code cleanup and saner handling of local branches

This patch cleans up some complicated code, and replaces it with a
cleaner version, using code from remote.[ch], which got extended a
little in the process.  This also enables us to fix two cases:

The earlier "fix" to setup tracking only when the original ref started
with "refs/remotes" is wrong.  You are absolutely allowed to use a
separate layout for your tracking branches.  The correct fix, of course,
is to set up tracking information only when there is a matching
remote.<nick>.fetch line containing a colon.

Another corner case was not handled properly.  If two remotes write to
the original ref, just warn the user and do not set up tracking.

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

Documentation/config.txt

@@ -309,10 +309,7 @@ branch.autosetupmerge::
 	so that gitlink:git-pull[1] will appropriately merge from that
 	remote branch.  Note that even if this option is not set,
 	this behavior can be chosen per-branch using the `--track`
-	and `--no-track` options.  This option can have values
-	'false' (never touch the configuration), 'all' (do this
-	for all branches), or 'true' (do this only when
-	branching from a remote tracking branch), and defaults to 'true'.
+	and `--no-track` options.  This option defaults to false.
 
 branch.<name>.remote::
 	When in branch <name>, it tells `git fetch` which remote to fetch.

Documentation/git-checkout.txt

@@ -52,9 +52,8 @@ OPTIONS
 	set up configuration so that git-pull will automatically
 	retrieve data from the remote branch.  Set the
 	branch.autosetupmerge configuration variable to true if you
-	want git-checkout and git-branch to behave as if
-	'--track' were given when you branch from a remote
-	tracking branch.
+	want git-checkout and git-branch to always behave as if
+	'--track' were given.
 
 --no-track::
 	When -b is given and a branch is created off a remote branch,