for-each-ref: let upstream/push optionally report the remote name

for-each-ref: let upstream/push optionally report the remote name

There are times when e.g. scripts want to know not only the name of the
upstream branch on the remote repository, but also the name of the
remote.

This patch offers the new suffix :remotename for the upstream and for
the push atoms, allowing to show exactly that. Example:

	$ cat .git/config
	...
	[remote "origin"]
		url = https://where.do.we.come/from
		fetch = refs/heads/*:refs/remote/origin/*
	[remote "hello-world"]
		url = https://hello.world/git
		fetch = refs/heads/*:refs/remote/origin/*
		pushURL = hello.world:git
		push = refs/heads/*:refs/heads/*
	[branch "master"]
		remote = origin
		pushRemote = hello-world
	...

	$ git for-each-ref \
	  --format='%(upstream) %(upstream:remotename) %(push:remotename)' \
	  refs/heads/master
	refs/remotes/origin/master origin hello-world

The implementation chooses *not* to DWIM the push remote if no explicit
push remote was configured; The reason is that it is possible to DWIM this
by using

	%(if)%(push:remotename)%(then)
		%(push:remotename)
	%(else)
		%(upstream:remotename)
	%(end)

while it would be impossible to "un-DWIM" the information in case the
caller is really only interested in explicit push remotes.

While `:remote` would be shorter, it would also be a bit more ambiguous,
and it would also shut the door e.g. for `:remoteref` (which would
obviously refer to the corresponding ref in the remote repository).

Note: the dashless, non-CamelCased form `:remotename` follows the
example of the `:trackshort` example.

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