git diff --no-index ... currently reads the index, during setup, when
calling gitmodules_config().  In the usual case this gives us some
performance drawbacks, but it's especially annoying if there is a broken
index file.

Avoid calling the unnecessary gitmodules_config() when the --no-index
option is given.  Also add a test to guard against similar breakages in the 
future.

Signed-off-by: Thomas Gummerer <t.gumme...@gmail.com>
---
 builtin/diff.c           | 13 +++++++++++--
 t/t4053-diff-no-index.sh |  6 ++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/builtin/diff.c b/builtin/diff.c
index adb93a9..47c0833 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c
@@ -257,7 +257,7 @@ int cmd_diff(int argc, const char **argv, const char 
*prefix)
        int blobs = 0, paths = 0;
        const char *path = NULL;
        struct blobinfo blob[2];
-       int nongit;
+       int nongit, no_index = 0;
        int result = 0;
 
        /*
@@ -282,9 +282,18 @@ int cmd_diff(int argc, const char **argv, const char 
*prefix)
         *
         * Other cases are errors.
         */
+       for (i = 1; i < argc; i++) {
+               if (!strcmp(argv[i], "--"))
+                       break;
+               if (!strcmp(argv[i], "--no-index")) {
+                       no_index = 1;
+                       break;
+               }
+       }
 
        prefix = setup_git_directory_gently(&nongit);
-       gitmodules_config();
+       if (!no_index)
+               gitmodules_config();
        git_config(git_diff_ui_config, NULL);
 
        init_revisions(&rev, prefix);
diff --git a/t/t4053-diff-no-index.sh b/t/t4053-diff-no-index.sh
index 979e983..a24ae4d 100755
--- a/t/t4053-diff-no-index.sh
+++ b/t/t4053-diff-no-index.sh
@@ -29,4 +29,10 @@ test_expect_success 'git diff --no-index relative path 
outside repo' '
        )
 '
 
+test_expect_success 'git diff --no-index with broken index' '
+       cd repo &&
+       echo broken >.git/index &&
+       test_expect_code 0 git diff --no-index a ../non/git/a
+'
+
 test_done
-- 
1.8.4.2

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