test-lib.sh: introduce test_commit() and test_merge() helpers

test-lib.sh: introduce test_commit() and test_merge() helpers

Often we just need to add a commit with a given (short) name, that will
be tagged with the same name.  Now, relatively complicated graphs can be
constructed easily and in a clear fashion:

	test_commit A &&
	test_commit B &&
	git checkout A &&
	test_commit C &&
	test_merge D B

will construct this graph:

	A - B
	  \   \
	    C - D

For simplicity, files whose name is the lower case version of the commit
message (to avoid a warning about ambiguous names) will be committed, with
the corresponding commit messages as contents.

If you need to provide a different file/different contents, you can use
the more explicit form

	test_commit $MESSAGE $FILENAME $CONTENTS

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>