Validate nicknames of remote branches to prohibit confusing ones

Validate nicknames of remote branches to prohibit confusing ones

The original problem was that the parsers for configuration files were
getting confused by seeing as nicknames remotes that involved
directory-changing characters. In particular, the branches config file
for ".." was particularly mystifying on platforms that can open
directories and read odd data from them.

The validation function was written by Junio Hamano (with a typo
corrected).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>