t/t1100-commit-tree-options.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>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1100-commit-tree-options.sh

@@ -35,11 +35,11 @@ test_expect_success \
      GIT_COMMITTER_NAME="Committer Name" \
      GIT_COMMITTER_EMAIL="committer@email" \
      GIT_COMMITTER_DATE="2005-05-26 23:30" \
-     TZ=GMT git commit-tree `cat treeid` >commitid 2>/dev/null'
+     TZ=GMT git commit-tree $(cat treeid) >commitid 2>/dev/null'
 
 test_expect_success \
     'read commit' \
-    'git cat-file commit `cat commitid` >commit'
+    'git cat-file commit $(cat commitid) >commit'
 
 test_expect_success \
     'compare commit' \