[build] Ignore .git in ninja copy

As part of the build the entire ninja source dir is copied to the build
dir and then built. When using git's fsmonitor feature the .git
directory contains a socket which causes the copytree to fail. With this
change .git is ignored entirely instead. Error:

```
Traceback (most recent call last):
  ...
  File "/Users/ksmiley/dev/oss-swift/swift/utils/swift_build_support/swift_build_support/build_script_invocation.py", line 69, in build_ninja
    ninja_build.build()
  File "/Users/ksmiley/dev/oss-swift/swift/utils/swift_build_support/swift_build_support/products/ninja.py", line 80, in build
    shell.copytree(self.source_dir, self.build_dir)
  File "/Users/ksmiley/dev/oss-swift/swift/utils/swift_build_support/swift_build_support/shell.py", line 193, in copytree
    shutil.copytree(src, dest)
  File "/Users/ksmiley/.pyenv/versions/3.10.2/lib/python3.10/shutil.py", line 556, in copytree
    return _copytree(entries=entries, src=src, dst=dst, symlinks=symlinks,
  File "/Users/ksmiley/.pyenv/versions/3.10.2/lib/python3.10/shutil.py", line 512, in _copytree
    raise Error(errors)
shutil.Error: [('/Users/ksmiley/dev/oss-swift/ninja/.git/fsmonitor--daemon.ipc', '/Users/ksmiley/dev/oss-swift/build/buildbot_osx/ninja-build/.git/fsmonitor--daemon.ipc', "[Errno 102] Operation not supported on socket: '/Users/ksmiley/dev/oss-swift/ninja/.git/fsmonitor--daemon.ipc'")]
```

Author: keith

utils/swift_build_support/swift_build_support/products/ninja.py

@@ -77,7 +77,7 @@ def build(self):
         # Ninja can only be built in-tree.  Copy the source tree to the build
         # directory.
         shell.rmtree(self.build_dir)
-        shell.copytree(self.source_dir, self.build_dir)
+        shell.copytree(self.source_dir, self.build_dir, ignore_pattern=".git")
         with shell.pushd(self.build_dir):
             shell.call([sys.executable, 'configure.py', '--bootstrap'],
                        env=env)

utils/swift_build_support/swift_build_support/shell.py

@@ -183,13 +183,14 @@ def rmtree(path, dry_run=None, echo=True):
         shutil.rmtree(path)
 
 
-def copytree(src, dest, dry_run=None, echo=True):
+def copytree(src, dest, dry_run=None, ignore_pattern=None, echo=True):
     dry_run = _coerce_dry_run(dry_run)
     if dry_run or echo:
         _echo_command(dry_run, ['cp', '-r', src, dest])
     if dry_run:
         return
-    shutil.copytree(src, dest)
+    ignore = shutil.ignore_patterns(ignore_pattern) if ignore_pattern else None
+    shutil.copytree(src, dest, ignore=ignore)
 
 
 def symlink(source, dest, dry_run=None, echo=True):