t9105-git-svn-commit-diff.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/t9105-git-svn-commit-diff.sh

@@ -18,8 +18,8 @@ test_expect_success 'initialize repo' '
 	git commit -a -m "another"
 	'
 
-head=`git rev-parse --verify HEAD^0`
+head=$(git rev-parse --verify HEAD^0)
-prev=`git rev-parse --verify HEAD^1`
+prev=$(git rev-parse --verify HEAD^1)
 
 # the internals of the commit-diff command are the same as the regular
 # commit, so only a basic test of functionality is needed since we've