git-gui/po/glossary/txt-to-pot.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: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

po/glossary/txt-to-pot.sh

@@ -11,7 +11,7 @@
 if [ $# -eq 0 ]
 then
 	cat <<!
-Usage: `basename $0` git-gui-glossary.txt > git-gui-glossary.pot
+Usage: $(basename $0) git-gui-glossary.txt > git-gui-glossary.pot
 !
 	exit 1;
 fi
@@ -33,7 +33,7 @@ cat <<!
 msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
-"POT-Creation-Date: `date +'%Y-%m-%d %H:%M%z'`\n"
+"POT-Creation-Date: $(date +'%Y-%m-%d %H:%M%z')\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"