sync: introduce --force-checkout

In some cases (e.g. in a CI system), it's desirable to be able to
instruct repo to force checkout. This flag passes --force flag to `git
checkout` operations.

Bug: b/327624021
Change-Id: I579edda546fb8147c4e1a267e2605fcf6e597421
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/411518
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: George Engelbrecht <engeg@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>

project.py

@@ -1515,6 +1515,7 @@ class Project:
         self,
         syncbuf,
         force_sync=False,
+        force_checkout=False,
         submodules=False,
         errors=None,
         verbose=False,
@@ -1602,7 +1603,7 @@ class Project:
                     syncbuf.info(self, "discarding %d commits", len(lost))
 
             try:
-                self._Checkout(revid, quiet=True)
+                self._Checkout(revid, force_checkout=force_checkout, quiet=True)
                 if submodules:
                     self._SyncSubmodules(quiet=True)
             except GitError as e:
@@ -2857,10 +2858,12 @@ class Project:
         except OSError:
             return False
 
-    def _Checkout(self, rev, quiet=False):
+    def _Checkout(self, rev, force_checkout=False, quiet=False):
         cmd = ["checkout"]
         if quiet:
             cmd.append("-q")
+        if force_checkout:
+            cmd.append("-f")
         cmd.append(rev)
         cmd.append("--")
         if GitCommand(self, cmd).Wait() != 0:

subcmds/sync.py

@@ -278,6 +278,11 @@ directories if they have previously been linked to a different
 object directory. WARNING: This may cause data to be lost since
 refs may be removed when overwriting.
 
+The --force-checkout option can be used to force git to switch revs even if the
+index or the working tree differs from HEAD, and if there are untracked files.
+WARNING: This may cause data to be lost since uncommitted changes may be
+removed.
+
 The --force-remove-dirty option can be used to remove previously used
 projects with uncommitted changes. WARNING: This may cause data to be
 lost since uncommitted changes may be removed with projects that no longer
@@ -375,6 +380,14 @@ later is required to fix a server side protocol bug.
             "point to a different object directory. WARNING: this "
             "may cause loss of data",
         )
+        p.add_option(
+            "--force-checkout",
+            dest="force_checkout",
+            action="store_true",
+            help="force checkout even if it results in throwing away "
+            "uncommitted modifications. "
+            "WARNING: this may cause loss of data",
+        )
         p.add_option(
             "--force-remove-dirty",
             dest="force_remove_dirty",
@@ -991,12 +1004,17 @@ later is required to fix a server side protocol bug.
 
         return _FetchMainResult(all_projects)
 
-    def _CheckoutOne(self, detach_head, force_sync, verbose, project):
+    def _CheckoutOne(
+        self, detach_head, force_sync, force_checkout, verbose, project
+    ):
         """Checkout work tree for one project
 
         Args:
             detach_head: Whether to leave a detached HEAD.
-            force_sync: Force checking out of the repo.
+            force_sync: Force checking out of .git directory (e.g. overwrite
+            existing git directory that was previously linked to a different
+            object directory).
+            force_checkout: Force checking out of the repo content.
             verbose: Whether to show verbose messages.
             project: Project object for the project to checkout.
 
@@ -1011,7 +1029,11 @@ later is required to fix a server side protocol bug.
         errors = []
         try:
             project.Sync_LocalHalf(
-                syncbuf, force_sync=force_sync, errors=errors, verbose=verbose
+                syncbuf,
+                force_sync=force_sync,
+                force_checkout=force_checkout,
+                errors=errors,
+                verbose=verbose,
             )
             success = syncbuf.Finish()
         except GitError as e:
@@ -1082,6 +1104,7 @@ later is required to fix a server side protocol bug.
                     self._CheckoutOne,
                     opt.detach_head,
                     opt.force_sync,
+                    opt.force_checkout,
                     opt.verbose,
                 ),
                 projects,