git-tag.sh: use the $( ... ) construct for command substitution

The Git CodingGuidelines prefer the $(...) construct for command
substitution instead of using the backquotes `...`.

The backquoted form is the traditional method for command
substitution, and is supported by POSIX.  However, all but the
simplest uses become complicated quickly.  In particular, embedded
command substitutions and/or the use of double quotes require
careful escaping with the backslash character.

The patch was generated by:

for _f in $(find . -name "*.sh")
do
   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-tag.sh

@@ -156,7 +156,7 @@ prev=0000000000000000000000000000000000000000
 if git show-ref --verify --quiet -- "refs/tags/$name"
 then
     test -n "$force" || die "tag '$name' already exists"
-    prev=`git rev-parse "refs/tags/$name"`
+    prev=$(git rev-parse "refs/tags/$name")
 fi
 shift
 git check-ref-format "tags/$name" ||