t3700 broken on pu on Cygwin

[Adam Dinwoodie <adam@dinwoodie.org>]

The t3700-add.sh test is currently failing on the pu branch on Cygwin.
To my surprise, the problem appears to have been introduced by a merge,
867fa1d6a.  Both parents of that merge have the test succeeding, but
it's failing on that merge commit.

Failing test output below:

    $ ./t3700-add.sh -i --verbose-only=41 -x
    Initialized empty Git repository in /home/add/vcs/git/x64/t/trash directory.t3700-add/.git/
    ok 1 - Test of git add
    ok 2 - Post-check that foo is in the index
    ok 3 - Test that "git add -- -q" works
    ok 4 - git add: Test that executable bit is not used if core.filemode=0
    ok 5 - git add: filemode=0 should not get confused by symlink
    ok 6 - git update-index --add: Test that executable bit is not used...
    ok 7 - git add: filemode=0 should not get confused by symlink
    ok 8 - git update-index --add: Test that executable bit is not used...
    ok 9 - .gitignore test setup
    ok 10 - .gitignore is honored
    ok 11 - error out when attempting to add ignored ones without -f
    ok 12 - error out when attempting to add ignored ones without -f
    ok 13 - error out when attempting to add ignored ones but add others
    ok 14 - add ignored ones with -f
    ok 15 - add ignored ones with -f
    ok 16 - add ignored ones with -f
    ok 17 - .gitignore with subdirectory
    ok 18 - check correct prefix detection
    ok 19 - git add with filemode=0, symlinks=0, and unmerged entries
    ok 20 - git add with filemode=0, symlinks=0 prefers stage 2 over stage 1
    ok 21 - git add --refresh
    ok 22 - git add --refresh with pathspec
    ok 23 # skip git add should fail atomically upon an unreadable file (missing SANITY of POSIXPERM,SANITY)
    ok 24 # skip git add --ignore-errors (missing SANITY of POSIXPERM,SANITY)
    ok 25 # skip git add (add.ignore-errors) (missing SANITY of POSIXPERM,SANITY)
    ok 26 # skip git add (add.ignore-errors = false) (missing SANITY of POSIXPERM,SANITY)
    ok 27 # skip --no-ignore-errors overrides config (missing SANITY of POSIXPERM,SANITY)
    ok 28 # skip git add 'fo\[ou\]bar' ignores foobar (missing BSLASHPSPEC)
    ok 29 - git add to resolve conflicts on otherwise ignored path
    ok 30 - "add non-existent" should fail
    ok 31 - git add -A on empty repo does not error out
    ok 32 - "git add ." in empty repo
    ok 33 - git add --dry-run of existing changed file
    ok 34 - git add --dry-run of non-existing file
    ok 35 - git add --dry-run of an existing file output
    ok 36 - git add --dry-run --ignore-missing of non-existing file
    ok 37 - git add --dry-run --ignore-missing of non-existing file output
    ok 38 - git add empty string should fail
    ok 39 - git add --chmod=[+-]x stages correctly
    ok 40 - git add --chmod=+x with symlinks

    expecting success:
    		git reset --hard &&
    		echo foo >foo3 &&
    		git add foo3 &&
    		git add --chmod=+x foo3 &&
    		test_mode_in_index 100755 foo3 &&
    		echo foo >xfoo3 &&
    		chmod 755 xfoo3 &&
    		git add xfoo3 &&
    		git add --chmod=-x xfoo3 &&
    		test_mode_in_index 100644 xfoo3

    ++ git reset --hard
    HEAD is now at d12df1f commit all
    ++ echo foo
    ++ git add foo3
    ++ git add --chmod=+x foo3
    ++ test_mode_in_index 100755 foo3
    ++ case "$(git ls-files -s "$2")" in
    +++ git ls-files -s foo3
    ++ echo pass
    pass
    ++ echo foo
    ++ chmod 755 xfoo3
    ++ git add xfoo3
    ++ git add --chmod=-x xfoo3
    cannot chmod 'xfoo3'++ test_mode_in_index 100644 xfoo3
    ++ case "$(git ls-files -s "$2")" in
    +++ git ls-files -s xfoo3
    ++ echo fail
    fail
    ++ git ls-files -s xfoo3
    120000 c5c4ca97a3a080c32920941b665e94a997901491 0       xfoo3
    ++ return 1
    + test_eval_ret_=1
    + want_trace
    + test t = t
    + test t = t
    + set +x
    error: last command exited with $?=1
    not ok 41 - git add --chmod=[+-]x changes index with already added file
    #
    #               git reset --hard &&
    #               echo foo >foo3 &&
    #               git add foo3 &&
    #               git add --chmod=+x foo3 &&
    #               test_mode_in_index 100755 foo3 &&
    #               echo foo >xfoo3 &&
    #               chmod 755 xfoo3 &&
    #               git add xfoo3 &&
    #               git add --chmod=-x xfoo3 &&
    #               test_mode_in_index 100644 xfoo3
    #