contrib/completion: escape the forward slash in __git_match_ctag

The current definition results in an incorrect expansion of the term under zsh. For instance "/^${1////\\/}/" under zsh with the argument "hi" results in: /^/\/h/\/i/ This results in an output similar to this when trying to complete `git grep chartab` under zsh: :: git grep chartabawk: cmd. line:1: /^/\/c/\/h/\/a/\/r/\/t/\/a/\/b/ { print $1 } awk: cmd. line:1: ^ backslash not last character on line awk: cmd. line:1: /^/\/c/\/h/\/a/\/r/\/t/\/a/\/b/ { print $1 } awk: cmd. line:1: ^ syntax error Leaving the prompt in a goofy state until the user hits a key. Escaping the literal / in the parameter expansion (using "/^${1//\//\\/}/") results in: /^chartab/ allowing the completion to work correctly. This formulation also works under bash. Signed-off-by: John Szakmeister <john@szakmeister.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>