Fun with cp -R error when both foo and foo.exe exist

[Dan Kegel <dank@kegel.com>]

Everybody who uses cygwin knows that if foo and foo.exe exist, you can do
  cp foo bar
  cp foo.exe bar.exe
but not
  cp foo.exe bar.exe
  cp foo bar
because of the unavoidable magic in cygwin that lets you execute foo
when you really mean foo.exe.

Well, it just bit me again during a buildbot try build of electron.
Here's the scenario:

 argv: ['git', '--version']
 argv: ['rm', '-rf',
'/home/buildbot/slave-state/electron-oblong-1-7-x-try-win10/build']
 argv: ['git', 'fetch', '-t', 'git@gitlab.foo.com:platform/electron.git']
 argv: ['git', 'fetch', '-t',
'git@gitlab.foo.com:jisorce/electron.git', 'window_infos']
 argv: ['git', 'reset', '--hard', 'FETCH_HEAD', '--']
 argv: ['git', 'checkout', '-B', 'window_infos']
 argv: ['git', 'submodule', 'sync']
 argv: ['git', 'submodule', 'update', '--init', '--recursive',
'--force', '--checkout']
 argv: ['cp', '-R', '-P', '-p', '-v',
'/home/buildbot/slave-state/electron-oblong-1-7-x-try-win10/source',
'/home/buildbot/slave-state/electron-oblong-1-7-x-try-win10/build']
cp: cannot create regular file
'/home/buildbot/slave-state/electron-oblong-1-7-x-try-win10/build/vendor/libchromiumcontent/vendor/depot_tools/ninja':
File exists

Maybe the order of files reported via the system call cp uses to
expand a directory was unfortunate after that git operation.

The workaround was to nuke the source directory, thus forcing a clean
checkout.  (And alas that means I don't have the directory to inspect
anymore.)

A nice workaround might be for the cygwin version of cp could arrange
to wait to create .exe files until after any potential non-suffixed
file has been processed... not sure how easy that would be.

Apologies if this has already been discussed.
- Dan

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