t1300: demonstrate failure when renaming sections with long lines

When renaming a configuration section which has an entry whose length
exceeds the size of our buffer in config.c's implementation of
`git_config_copy_or_rename_section_in_file()`, Git will incorrectly
form a new configuration section with part of the data in the section
being removed.

In this instance, our first configuration file looks something like:

    [b]
      c = d <spaces> [a] e = f
    [a]
      g = h

Here, we have two configuration values, "b.c", and "a.g". The value "[a]
e = f" belongs to the configuration value "b.c", and does not form its
own section.

However, when renaming the section 'a' to 'xyz', Git will write back
"[xyz]\ne = f", but "[xyz]" is still attached to the value of "b.c",
which is why "e = f" on its own line becomes a new entry called "b.e".

A slightly different example embeds the section being renamed within
another section.

Demonstrate this failure in a test in t1300, which we will fix in the
following commit.

Co-authored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t1300-config.sh

@@ -613,6 +613,26 @@ test_expect_success 'renaming to bogus section is rejected' '
 	test_must_fail git config --rename-section branch.zwei "bogus name"
 '
 
+test_expect_failure 'renaming a section with a long line' '
+	{
+		printf "[b]\\n" &&
+		printf "  c = d %1024s [a] e = f\\n" " " &&
+		printf "[a] g = h\\n"
+	} >y &&
+	git config -f y --rename-section a xyz &&
+	test_must_fail git config -f y b.e
+'
+
+test_expect_failure 'renaming an embedded section with a long line' '
+	{
+		printf "[b]\\n" &&
+		printf "  c = d %1024s [a] [foo] e = f\\n" " " &&
+		printf "[a] g = h\\n"
+	} >y &&
+	git config -f y --rename-section a xyz &&
+	test_must_fail git config -f y foo.e
+'
+
 cat >> .git/config << EOF
   [branch "zwei"] a = 1 [branch "vier"]
 EOF