remote: remove branch->merge_name and fix branch_release()

The branch structure has both branch->merge_name and branch->merge for
tracking the merge information. The former is allocated by add_merge()
and stores the names read from the configuration file. The latter is
allocated by set_merge() which is called by branch_get() when an
external caller requests a branch.

This leads to the confusing situation where branch->merge_nr tracks both
the size of branch->merge (once its allocated) and branch->merge_name.
The branch_release() function incorrectly assumes that branch->merge is
always set when branch->merge_nr is non-zero, and can potentially crash
if read_config() is called without branch_get() being called on every
branch.

In addition, branch_release() fails to free some of the memory
associated with the structure including:

 * Failure to free the refspec_item containers in branch->merge[i]
 * Failure to free the strings in branch->merge_name[i]
 * Failure to free the branch->merge_name parent array.

The set_merge() function sets branch->merge_nr to 0 when there is no
valid remote_name, to avoid external callers seeing a non-zero merge_nr
but a NULL merge array. This results in failure to release most of the
merge data as well.

These issues could be fixed directly, and indeed I initially proposed
such a change at [1] in the past. While this works, there was some
confusion during review because of the inconsistencies.

Instead, its time to clean up the situation properly. Remove
branch->merge_name entirely. Instead, allocate branch->merge earlier
within add_merge() instead of within set_merge(). Instead of having
set_merge() copy from merge_name[i] to merge[i]->src, just have
add_merge() directly initialize merge[i]->src.

Modify the add_merge() to call xstrdup() itself, instead of having
the caller of add_merge() do so. This makes it more obvious which code
owns the memory.

Update all callers which use branch->merge_name[i] to use
branch->merge[i]->src instead.

Add a merge_clear() function which properly releases all of the
merge-related memory, and which sets branch->merge_nr to zero. Use this
both in branch_release() and in set_merge(), fixing the leak when
set_merge() finds no valid remote_name.

Add a set_merge variable to the branch structure, which indicates
whether set_merge() has been called. This replaces the previous use of a
NULL check against the branch->merge array.

With these changes, the merge array is always allocated when merge_nr is
non-zero.

This use of refspec_item to store the names should be safe. External
callers should be using branch_get() to obtain a pointer to the branch,
which will call set_merge(), and the callers internal to remote.c
already handle the partially initialized refpsec_item structure safely.

This end result is cleaner, and avoids duplicating the merge names
twice.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Link: [1] https://lore.kernel.org/git/20250617-jk-submodule-helper-use-url-v2-1-04cbb003177d@gmail.com/
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.h

@@ -315,8 +315,8 @@ struct branch {
 
 	char *pushremote_name;
 
-	/* An array of the "merge" lines in the configuration. */
-	const char **merge_name;
+	/* True if set_merge() has been called to finalize the merge array */
+	int set_merge;
 
 	/**
 	 * An array of the struct refspecs used for the merge lines. That is,