index: use the checksum to check whether it's been modified

We currently use a timetamp to check whether an index file has been
modified since we last read it, but this is racy. If two updates happen
in the same second and we read after the first one, we won't detect the
second one.

Instead read the SHA-1 checksum of the file, which are its last 20 bytes which
gives us a sure-fire way to detect whether the file has changed since we
last read it.

As we're now keeping track of it, expose an accessor to this data.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/index.h b/src/index.h
index 0f6f4e86e..615d703c4 100644
--- a/src/index.h
+++ b/src/index.h
@@ -22,6 +22,7 @@ struct git_index {
 
 	char *index_file_path;
 	git_futils_filestamp stamp;
+	git_oid checksum;   /* checksum at the end of the file */
 
 	git_vector entries;