attr: teach "--attr-source=<tree>" global option to "git"

Earlier, 47cfc9bd (attr: add flag `--source` to work with tree-ish,
2023-01-14) taught "git check-attr" the "--source=<tree>" option to
allow it to read attribute files from a tree-ish, but did so only
for the command.  Just like "check-attr" users wanted a way to use
attributes from a tree-ish and not from the working tree files,
users of other commands (like "git diff") would benefit from the
same.

Undo most of the UI change the commit made, while keeping the
internal logic to read attributes from a given tree-ish. Expose the
internal logic via a new "--attr-source=<tree>" command line option
given to "git", so that it can be used with any git command that
runs as part of the main git process.

Additionally, add an environment variable GIT_ATTR_SOURCE that is set
when --attr-source is passed in, so that subprocesses use the same value
for the attributes source tree.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 4 deletions

diff --git a/attr.h b/attr.h
index 9884ea2bc6..676bd17ce2 100644
--- a/attr.h
+++ b/attr.h
@@ -45,7 +45,7 @@
  * const char *path;
  *
  * setup_check();
- * git_check_attr(&the_index, tree_oid, path, check);
+ * git_check_attr(&the_index, path, check);
  * ------------
  *
  * - Act on `.value` member of the result, left in `check->items[]`:
@@ -120,7 +120,6 @@
 #define ATTR_MAX_FILE_SIZE (100 * 1024 * 1024)
 
 struct index_state;
-struct object_id;
 
 /**
  * An attribute is an opaque object that is identified by its name. Pass the
@@ -136,6 +135,12 @@ struct all_attrs_item;
 struct attr_stack;
 
 /*
+ * The textual object name for the tree-ish used by git_check_attr()
+ * to read attributes from (instead of from the working tree).
+ */
+void set_git_attr_source(const char *);
+
+/*
  * Given a string, return the gitattribute object that
  * corresponds to it.
  */
@@ -203,14 +208,14 @@ void attr_check_free(struct attr_check *check);
 const char *git_attr_name(const struct git_attr *);
 
 void git_check_attr(struct index_state *istate,
-		    const struct object_id *tree_oid, const char *path,
+		    const char *path,
 		    struct attr_check *check);
 
 /*
  * Retrieve all attributes that apply to the specified path.
  * check holds the attributes and their values.
  */
-void git_all_attrs(struct index_state *istate, const struct object_id *tree_oid,
+void git_all_attrs(struct index_state *istate,
 		   const char *path, struct attr_check *check);
 
 enum git_attr_direction {