Add test case for merge-base.

Although it was shown that the "full contamination" was not really full
during the list discussion, the series improves things without incurring
extra parsing cost, and here is a test to check that.

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

t/t6010-merge-base.sh

@@ -0,0 +1,59 @@
+#!/bin/sh
+#
+# Copyright (c) 2005 Junio C Hamano
+#
+
+test_description='Merge base computation.
+'
+
+. ./test-lib.sh
+
+T=$(git-write-tree)
+
+M=1130000000
+Z=+0000
+
+export GIT_COMMITTER_EMAIL=git@comm.iter.xz
+export GIT_COMMITTER_NAME='C O Mmiter'
+export GIT_AUTHOR_NAME='A U Thor'
+export GIT_AUTHOR_EMAIL=git@au.thor.xz
+
+doit() {
+	OFFSET=$1; shift
+	NAME=$1; shift
+	PARENTS=
+	for P
+	do
+		PARENTS="${PARENTS}-p $P "
+	done
+	GIT_COMMITTER_DATE="$(($M + $OFFSET)) $Z"
+	GIT_AUTHOR_DATE=$GIT_COMMITTER_DATE
+	export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
+	commit=$(echo $NAME | git-commit-tree $T $PARENTS)
+	echo $commit >.git/refs/tags/$NAME
+	echo $commit
+}
+
+# Setup...
+E=$(doit 5 E)
+D=$(doit 4 D $E)
+F=$(doit 6 F $E)
+C=$(doit 3 C $D)
+B=$(doit 2 B $C)
+A=$(doit 1 A $B)
+G=$(doit 7 G $B $E)
+H=$(doit 8 H $A $F)
+
+test_expect_success 'compute merge-base (single)' \
+    'MB=$(git-merge-base G H) &&
+     expr "$(git-name-rev "$MB")" : "[0-9a-f]* B"'
+
+test_expect_success 'compute merge-base (all)' \
+    'MB=$(git-merge-base --all G H) &&
+     expr "$(git-name-rev "$MB")" : "[0-9a-f]* B"'
+
+test_expect_success 'compute merge-base with show-branch' \
+    'MB=$(git-show-branch --merge-base G H) &&
+     expr "$(git-name-rev "$MB")" : "[0-9a-f]* B"'
+
+test_done