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Merge branch 'mg/doc-bisect-tweak-worktree' into maint

* mg/doc-bisect-tweak-worktree:
git-bisect.txt: example for bisecting with hot-fix
git-bisect.txt: streamline run presentation

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Documentation/git-bisect.txt

 $ git bisect run make test           # "make test" builds and tests
 ------------
 
-* Automatically bisect a broken test suite:
+* Automatically bisect a broken test case:
 +
 ------------
 $ cat ~/test.sh
 #!/bin/sh
-make || exit 125                   # this skips broken builds
-make test                          # "make test" runs the test suite
-$ git bisect start v1.3 v1.1 --    # v1.3 is bad, v1.1 is good
+make || exit 125                     # this skips broken builds
+~/check_test_case.sh                 # does the test case pass?
+$ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
 $ git bisect run ~/test.sh
 ------------
 +
 Here we use a "test.sh" custom script. In this script, if "make"
 fails, we skip the current commit.
+"check_test_case.sh" should "exit 0" if the test case passes,
+and "exit 1" otherwise.
 +
-It is safer to use a custom script outside the repository to prevent
-interactions between the bisect, make and test processes and the
-script.
-+
-"make test" should "exit 0", if the test suite passes, and
-"exit 1" otherwise.
+It is safer if both "test.sh" and "check_test_case.sh" are
+outside the repository to prevent interactions between the bisect,
+make and test processes and the scripts.
 
-* Automatically bisect a broken test case:
+* Automatically bisect with temporary modifications (hot-fix):
 +
 ------------
 $ cat ~/test.sh
 #!/bin/sh
-make || exit 125                     # this skips broken builds
-~/check_test_case.sh                 # does the test case passes ?
-$ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
-$ git bisect run ~/test.sh
+
+# tweak the working tree by merging the hot-fix branch
+# and then attempt a build
+if	git merge --no-commit hot-fix &&
+	make
+then
+	# run project specific test and report its status
+	~/check_test_case.sh
+	status=$?
+else
+	# tell the caller this is untestable
+	status=125
+fi
+
+# undo the tweak to allow clean flipping to the next commit
+git reset --hard
+
+# return control
+exit $status
 ------------
 +
-Here "check_test_case.sh" should "exit 0" if the test case passes,
-and "exit 1" otherwise.
-+
-It is safer if both "test.sh" and "check_test_case.sh" scripts are
-outside the repository to prevent interactions between the bisect,
-make and test processes and the scripts.
+This applies modifications from a hot-fix branch before each test run,
+e.g. in case your build or test environment changed so that older
+revisions may need a fix which newer ones have already. (Make sure the
+hot-fix branch is based off a commit which is contained in all revisions
+which you are bisecting, so that the merge does not pull in too much, or
+use `git cherry-pick` instead of `git merge`.)
 
-* Automatically bisect a broken test suite:
+* Automatically bisect a broken test case:
 +
 ------------
 $ git bisect start HEAD HEAD~10 --   # culprit is among the last 10
 $ git bisect run sh -c "make || exit 125; ~/check_test_case.sh"
 ------------
 +
-Does the same as the previous example, but on a single line.
+This shows that you can do without a run script if you write the test
+on a single line.
 
 Author
 ------