add experimental git worktree support

This provides initial support for using git worktrees internally
instead of our own ad-hoc symlink tree.  It's been lightly tested
which is why it's not currently exposed via --help.

When people opt-in to worktrees in an existing repo client checkout,
no projects are migrated.  Instead, only new projects will use the
worktree method.  This allows for limited testing/opting in without
having to completely blow things away or get a second checkout.

Bug: https://crbug.com/gerrit/11486
Change-Id: Ic3ff891b30940a6ba497b406b2a387e0a8517ed8
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254075
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -15,6 +15,8 @@
 # limitations under the License.
 
 from __future__ import print_function
+
+import errno
 import json
 import netrc
 from optparse import SUPPRESS_HELP
@@ -569,7 +571,8 @@ later is required to fix a server side protocol bug.
     gc_gitdirs = {}
     for project in projects:
       # Make sure pruning never kicks in with shared projects.
-      if len(project.manifest.GetProjectsWithName(project.name)) > 1:
+      if (not project.use_git_worktrees and
+          len(project.manifest.GetProjectsWithName(project.name)) > 1):
         print('%s: Shared project %s found, disabling pruning.' %
               (project.relpath, project.name))
         if git_require((2, 7, 0)):
@@ -637,13 +640,22 @@ later is required to fix a server side protocol bug.
     # Delete the .git directory first, so we're less likely to have a partially
     # working git repository around. There shouldn't be any git projects here,
     # so rmtree works.
+    dotgit = os.path.join(path, '.git')
+    # Try to remove plain files first in case of git worktrees.  If this fails
+    # for any reason, we'll fall back to rmtree, and that'll display errors if
+    # it can't remove things either.
     try:
-      platform_utils.rmtree(os.path.join(path, '.git'))
+      platform_utils.remove(dotgit)
+    except OSError:
+      pass
+    try:
+      platform_utils.rmtree(dotgit)
     except OSError as e:
-      print('Failed to remove %s (%s)' % (os.path.join(path, '.git'), str(e)), file=sys.stderr)
-      print('error: Failed to delete obsolete path %s' % path, file=sys.stderr)
-      print('       remove manually, then run sync again', file=sys.stderr)
-      return 1
+      if e.errno != errno.ENOENT:
+        print('error: %s: %s' % (dotgit, str(e)), file=sys.stderr)
+        print('error: %s: Failed to delete obsolete path; remove manually, then '
+              'run sync again' % (path,), file=sys.stderr)
+        return 1
 
     # Delete everything under the worktree, except for directories that contain
     # another git project