t4150: fix broken test for am --scissors

t4150: fix broken test for am --scissors

Tests for "git am --[no-]scissors" [1] work in the following way:

 1. Create files with commit messages
 2. Use these files to create expected commits
 3. Generate eml file with patch from expected commits
 4. Create commits using git am with these eml files
 5. Compare these commits with expected

The test for "git am --scissors" is supposed to take an e-mail with a
scissors line and in-body "Subject:" header and demonstrate that the
subject line from the e-mail itself is overridden by the in-body header
and that only text below the scissors line is included in the commit
message of the commit created by the invocation of "git am --scissors".
However, the setup of the test incorrectly uses a commit without the
scissors line and without the in-body header in the commit message,
producing eml file not suitable for testing of "git am --scissors".

This can be checked by intentionally breaking is_scissors_line function
in mailinfo.c, for example, by changing string ">8", which is used by
the test. With such change the test should fail, but does not.

Fix broken test by generating eml file with scissors line and in-body
header "Subject:". Since the two tests for --scissors and --no-scissors
options are there to test cutting or keeping the commit message, update
both tests to change the test file in the same way, which allows us to
generate only one eml file to be passed to git am. To clarify the
intention of the test, give files and tags more explicit names.

[1]: introduced in bf72ac17d (t4150: tests for am --[no-]scissors,
     2015-07-19)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Reviewed-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>