blob: allow blob filtering to ignore system gitattributes

Introduce `GIT_BLOB_FILTER_NO_SYSTEM_ATTRIBUTES`, which tells
`git_blob_filter` to ignore the system-wide attributes file, usually
`/etc/gitattributes`.

This simply passes the appropriate flag to the attribute loading code.

include/git2/blob.h

@@ -102,6 +102,12 @@ GIT_EXTERN(git_off_t) git_blob_rawsize(const git_blob *blob);
 typedef enum {
 	/** When set, filters will not be applied to binary files. */
 	GIT_BLOB_FILTER_CHECK_FOR_BINARY = (1 << 0),
+
+	/**
+	 * When set, filters will not load configuration from the
+	 * system-wide `gitattributes` in `/etc` (or system equivalent).
+	 */
+	GIT_BLOB_FILTER_NO_SYSTEM_ATTRIBUTES = (1 << 1),
 } git_blob_filter_flag_t;
 
 /**

src/blob.c

@@ -425,6 +425,9 @@ int git_blob_filter(
 	    git_blob_is_binary(blob))
 		return 0;
 
+	if ((opts.flags & GIT_BLOB_FILTER_NO_SYSTEM_ATTRIBUTES) != 0)
+		flags |= GIT_FILTER_NO_SYSTEM_ATTRIBUTES;
+
 	if (!(error = git_filter_list_load(
 			&fl, git_blob_owner(blob), blob, path,
 			GIT_FILTER_TO_WORKTREE, flags))) {