remote: write correct fetch spec when renaming remote 'remote'

remote: write correct fetch spec when renaming remote 'remote'

When renaming a remote whose name is contained in a configured fetch
refspec for that remote, we currently replace the first occurrence of
the remote name in the refspec. This is correct in most cases, but
breaks if the remote name occurs in the fetch refspec before the
expected place. For example, we currently change

[remote "remote"]
	url = git://git.kernel.org/pub/scm/git/git.git
	fetch = +refs/heads/*:refs/remotes/remote/*

into

[remote "origin"]
	url = git://git.kernel.org/pub/scm/git/git.git
	fetch = +refs/heads/*:refs/origins/remote/*

Reduce the risk of changing incorrect sections of the refspec by
matching the entire ":refs/remotes/<name>/" instead of just "<name>".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>