fast-export: Allow pruned-references in mark file

fast-export can fail because of some pruned-reference when importing a
mark file.

The problem happens in the following scenario:

    $ git fast-export --export-marks=MARKS master
    (rewrite master)
    $ git prune
    $ git fast-export --import-marks=MARKS master

This might fail if some references have been removed by prune
because some marks will refer to no longer existing commits.
git-fast-export will not need these objects anyway as they were no
longer reachable.

We still need to update last_numid so we don't change the mapping
between marks and objects for remote-helpers.
Unfortunately, the mark file should not be rewritten without lost marks
if no new objects has been exported, as we could lose track of the last
last_numid.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/Documentation/git-fast-export.txt b/Documentation/git-fast-export.txt
index d6487e1..feab7a3 100644
--- a/Documentation/git-fast-export.txt
+++ b/Documentation/git-fast-export.txt
@@ -66,6 +66,8 @@ produced incorrect results if you gave these options.
 	incremental runs.  As <file> is only opened and truncated
 	at completion, the same path can also be safely given to
 	\--import-marks.
+	The file will not be written if no new object has been
+	marked/exported.
 
 --import-marks=<file>::
 	Before processing any input, load the marks specified in