`magit-rename-branch': use old name as initial input for new name

When renaming branches, one often wants to change only a small portion
of the name (e.g. removing a prefix like "wip/"), so when asking for the
new name, use the old name as initial input instead of making us type
the whole branch name again.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-rename-branch': do some error checking

Avoid needlessly calling `git-branch(1)` with arguments which
are known to be invalid/pointless.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-rename-remote': use old name as initial input for new name

When renaming remotes, one often wants to change only a small portion
of the name (e.g. to correct a typo), so when asking for the new name,
use the old name as initial input instead of making us type the whole
remote name again.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-rename-remote': do some error checking

Avoid needlessly calling `git-remote(1) rename` with arguments which
are known to be invalid/pointless.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #932 from praet/rename

Improving `magit-rename-*` functions