Set +x in index in added-file-changed-content-and-mode

This fixes the second `gix-merge::merge tree::run_baseline` failure
in which the baseline fixture `added-file-changed-content-and-mode`
produced the wrong results when run on Windows, due to `chmod +x`
having no effect (but reporting success) when run in Git Bash.

The failure that this fixes, which is visible since the fix for
the first baseline case failure was fixed in the preceding commit,
is:

    --- STDERR:              gix-merge::merge tree::run_baseline ---
    failed to extract 'tests\fixtures\generated-archives\tree-baseline.tar': Ignoring archive at 'tests\fixtures\generated-archives\tree-baseline.tar' as GIX_TEST_IGNORE_ARCHIVES is set.
    thread 'tree::run_baseline' panicked at gix-merge\tests\merge\tree\mod.rs:93:17:
    assertion failed: `(left == right)`: added-file-changed-content-and-mode-A-B-reversed: tree mismatch: We improve on executable bit handling, but loose on diff quality as we are definitely missing some tweaks
    [
        Conflict {
            resolution: Ok(
                OursModifiedTheirsModifiedThenBlobContentMerge {
                    merged_blob: ContentMerge {
                        merged_blob_id: Sha1(7ad0b902fe3f9d6857ca2e02a84f17c3eae607d6),
                        resolution: Conflict,
                    },
                },
            ),
            ours: Addition {
                location: "new",
                relation: None,
                entry_mode: EntryMode(
                    33188,
                ),
                id: Sha1(ec046db5d8b05470e472330d75f419c81ed6c9d3),
            },
            theirs: Addition {
                location: "new",
                relation: None,
                entry_mode: EntryMode(
                    33188,
                ),
                id: Sha1(8a1218a1024a212bb3db30becd860315f9f3ac52),
            },
            map: Original,
        },
    ]
    added-file-changed-content-and-mode-A-B-reversed

    Diff < left / right > :
    <297247d
    >c5c706c
     ├── a:fd269b6
     │   └── x.f:100644:4406528 "original\n1\n2\n3\n4\n5\n"
    <└── new:100644:7ad0b90 "<<<<<<< B\noriginal\n1\n2\n3\n4\n5\n6\n=======\n1\n2\n3\n4\n5\n>>>>>>> A\n"
    >└── new:100644:c3e610f "<<<<<<< A\n1\n2\n3\n4\n5\n=======\noriginal\n1\n2\n3\n4\n5\n6\n>>>>>>> B\n"

See the previous commit for a general description. The situation
here differs because the file, `new`, is newly created at the same
time as its mode is set, so it is not already in the index. While
staging it and using the same approach used in the previous commit
would work, this instead adds `--chmod=+x` to the existing
`git add` commands to stage them with the desired permissions
initially, since here those commands immediately follow the
`chmod +x` commands.

To make this work (and so it is clear), this also changes the path
argument to refer to the specific file, rather than passing `.`,
which had alredy been done in one place. These commands are (and
were) only staging a single file, so this is sufficient.

It turns out that the baseline cases fixed in the preceding commit
and here are the only causes of failure for that test, so the test
is now passing, even on Windows with `GIX_TEST_IGNORE_ARCHIVES=1`.

Author: EliahKagan

gix-merge/tests/fixtures/tree-baseline.sh

@@ -530,13 +530,13 @@ git init added-file-changed-content-and-mode
   git checkout B
   write_lines original 1 2 3 4 5 6 >new
   chmod +x new
-  git add .
+  git add --chmod=+x new
   git commit -m "add new with content B and +x"
 
   git checkout expected
   echo -n $'<<<<<<< A\n1\n2\n3\n4\n5\n=======\noriginal\n1\n2\n3\n4\n5\n6\n>>>>>>> B\n' >new
   chmod +x new
-  git add new
+  git add --chmod=+x new
   git commit -m "Git has a better merge here, but that's due to better hunk handling/hunk splitting. We, however, consistently use +x"
 )