Teach update-index about --ignore-submodules

Like with the diff machinery, update-index should sometimes just
ignore submodules (e.g. to determine a clean state before a rebase).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -15,6 +15,7 @@ SYNOPSIS
 	     [--cacheinfo <mode> <object> <file>]\*
 	     [--chmod=(+|-)x]
 	     [--assume-unchanged | --no-assume-unchanged]
+	     [--ignore-submodules]
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin]
@@ -54,6 +55,10 @@ OPTIONS
         default behavior is to error out.  This option makes
         git-update-index continue anyway.
 
+--ignore-submodules:
+	Do not try to update submodules.  This option is only respected
+	when passed before --refresh.
+
 --unmerged::
         If --refresh finds unmerged changes in the index, the default
         behavior is to error out.  This option makes git-update-index

builtin-update-index.c

@@ -593,6 +593,10 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 				refresh_flags |= REFRESH_QUIET;
 				continue;
 			}
+			if (!strcmp(path, "--ignore-submodules")) {
+				refresh_flags |= REFRESH_IGNORE_SUBMODULES;
+				continue;
+			}
 			if (!strcmp(path, "--add")) {
 				allow_add = 1;
 				continue;

cache.h

@@ -388,6 +388,7 @@ extern void fill_stat_cache_info(struct cache_entry *ce, struct stat *st);
 #define REFRESH_UNMERGED	0x0002	/* allow unmerged */
 #define REFRESH_QUIET		0x0004	/* be quiet about it */
 #define REFRESH_IGNORE_MISSING	0x0008	/* ignore non-existent */
+#define REFRESH_IGNORE_SUBMODULES	0x0008	/* ignore submodules */
 extern int refresh_index(struct index_state *, unsigned int flags, const char **pathspec, char *seen);
 
 struct lock_file {

read-cache.c

@@ -942,13 +942,17 @@ int refresh_index(struct index_state *istate, unsigned int flags, const char **p
 	int allow_unmerged = (flags & REFRESH_UNMERGED) != 0;
 	int quiet = (flags & REFRESH_QUIET) != 0;
 	int not_new = (flags & REFRESH_IGNORE_MISSING) != 0;
+	int ignore_submodules = (flags & REFRESH_IGNORE_SUBMODULES) != 0;
 	unsigned int options = really ? CE_MATCH_IGNORE_VALID : 0;
 
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce, *new;
 		int cache_errno = 0;
 
 		ce = istate->cache[i];
+		if (ignore_submodules && S_ISGITLINK(ce->ce_mode))
+			continue;
+
 		if (ce_stage(ce)) {
 			while ((i < istate->cache_nr) &&
 			       ! strcmp(istate->cache[i]->name, ce->name))