scripts: equality test '==' is not portable.

On NetBSD 3 we trigger an error:

    [: ==: unexpected operator

Double-equal is accepted by bash built-in '[' and bash(1) suggests
using '=' for strict POSIX compliance (test(1) from coreutils does not
mention '==').  Eradicate their uses everywhere.

[jc: Somebody with a pseudonym kindly sent a message to let
 me know about the problem privately; I do not have access to a NetBSD
 box.]

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-resolve-script

@@ -36,19 +36,21 @@ if [ -z "$common" ]; then
 	die "Unable to find common commit between" $merge $head
 fi
 
-if [ "$common" == "$merge" ]; then
+case "$common" in
+"$merge")
 	echo "Already up-to-date. Yeeah!"
 	dropheads
 	exit 0
-fi
-if [ "$common" == "$head" ]; then
+	;;
+"$head")
 	echo "Updating from $head to $merge."
 	git-read-tree -u -m $head $merge || exit 1
 	echo $merge > "$GIT_DIR"/HEAD
 	git-diff-tree -p $head $merge | git-apply --stat
 	dropheads
 	exit 0
-fi
+	;;
+esac
 
 # Find an optimum merge base if there are more than one candidates.
 LF='

git-status-script

@@ -77,9 +77,9 @@ then
 #'
 fi
 
-if [ "$committable" == "0" ]
-then
+case "$committable" in
+0)
 	echo "nothing to commit"
 	exit 1
-fi
+esac
 exit 0

templates/hooks--update

@@ -16,10 +16,14 @@ then
 	git-rev-list --pretty "$3"
 else
 	$base=$(git-merge-base "$2" "$3")
-	if [ $base == "$2" ]; then
+	case "$base" in
+	"$2")
 		echo "New commits:"
-	else
+		;;
+	*)
 		echo "Rebased ref, commits from common ancestor:"
+		;;
+	esac
 fi
 git-rev-list --pretty "$3" "^$base"
 fi |