t/t5522-pull-symlink.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
	perl -i -pe 'BEGIN{undef $/;} s/`(.+?)`/\$(\1)/smg'  "${_f}"
done

and then carefully proof-read.

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

t/t5522-pull-symlink.sh

@@ -54,7 +54,7 @@ test_expect_success SYMLINKS 'pulling from real subdir' '
 # git rev-parse --show-cdup printed a path relative to
 # clone-repo/subdir/, not subdir-link/.  Git rev-parse --show-cdup
 # used the correct .git, but when the git pull shell script did
-# "cd `git rev-parse --show-cdup`", it ended up in the wrong
+# "cd $(git rev-parse --show-cdup)", it ended up in the wrong
 # directory.  A POSIX shell's "cd" works a little differently
 # than chdir() in C; "cd -P" is much closer to chdir().
 #