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>

t/README

@@ -212,6 +212,24 @@ library for your script to use.
    is to summarize successes and failures in the test script and
    exit with an appropriate error code.
 
+ - test_tick
+
+   Make commit and tag names consistent by setting the author and
+   committer times to defined stated.  Subsequent calls will
+   advance the times by a fixed amount.
+
+ - test_commit <message> [<filename> [<contents>]]
+
+   Creates a commit with the given message, committing the given
+   file with the given contents (default for both is to reuse the
+   message string), and adds a tag (again reusing the message
+   string as name).  Calls test_tick to make the SHA-1s
+   reproducible.
+
+ - test_merge <message> <commit-or-tag>
+
+   Merges the given rev using the given message.  Like test_commit,
+   creates a tag and calls test_tick before committing.
 
 Tips for Writing Tests
 ----------------------

t/test-lib.sh

@@ -193,6 +193,31 @@ test_tick () {
 	export GIT_COMMITTER_DATE GIT_AUTHOR_DATE
 }
 
+# Call test_commit with the arguments "<message> [<file> [<contents>]]"
+#
+# This will commit a file with the given contents and the given commit
+# message.  It will also add a tag with <message> as name.
+#
+# Both <file> and <contents> default to <message>.
+
+test_commit () {
+	file=${2:-$(echo "$1" | tr 'A-Z' 'a-z')}
+	echo "${3-$1}" > "$file" &&
+	git add "$file" &&
+	test_tick &&
+	git commit -m "$1" &&
+	git tag "$1"
+}
+
+# Call test_merge with the arguments "<message> <commit>", where <commit>
+# can be a tag pointing to the commit-to-merge.
+
+test_merge () {
+	test_tick &&
+	git merge -m "$1" "$2" &&
+	git tag "$1"
+}
+
 # You are not expected to call test_ok_ and test_failure_ directly, use
 # the text_expect_* functions instead.