Only use temp index if hunks are staged

For big repos reading HEAD's tree into the temp index can apparently
take a while, so we should skip creating it if we can. Also, if we use a
temp index for unstaged hunks, then the original index will be
out-of-sync with any hunks that were committed via the temp index.

If you hit this, `git restore --staged` or `git read-tree HEAD^{tree}`
can be used to read the new HEAD's tree into the index.

In tests, if no staged changes are explicitly expected via the
`staged_want` param, fail if anything is in the index after calling
main().

Fixes #16

git-autofixup

@@ -553,9 +553,11 @@ sub main {
         return 0;
     }
 
-    # Limit the tempfile's lifetime to the execution of main().
-    local $ENV{GIT_INDEX_FILE};  # Throw away ENV changes between main() calls.
-    my $tempfile = create_temp_index();
+    local $ENV{GIT_INDEX_FILE};  # Throw away changes between main() calls.
+    if (is_index_dirty()) {
+        # Limit the tempfile's lifetime to the execution of main().
+        my $tempfile = create_temp_index();
+    }
 
     for my $sha (@ordered_shas) {
         my $fixup_hunks = $hunks_for->{$sha};

t/autofixup.t

@@ -77,6 +77,7 @@ sub test_autofixup {
     my $staged = $args->{staged};
     my $log_want = defined($args->{log_want}) ? $args->{log_want}
                  : croak "wanted log output not given";
+    my $staged_want = $args->{staged_want};
     my $unstaged_want = $args->{unstaged_want};
     my $exit_code_want = $args->{exit_code};
     my $autofixup_opts = $args->{autofixup_opts} || [];
@@ -90,6 +91,7 @@ sub test_autofixup {
 
     my $exit_code_got;
     my $log_got;
+    my $staged_got;
     my $unstaged_got;
     my $orig_dir = getcwd();
     my $dir = File::Temp::tempdir(CLEANUP => 1);
@@ -126,6 +128,7 @@ sub test_autofixup {
         run("git --no-pager log --format='%h %s' ${upstream_rev}..");
         $exit_code_got = autofixup(@{$autofixup_opts}, $upstream_rev);
         $log_got = git_log(${pre_fixup_rev});
+        $staged_got = diff('--cached');
         if (defined($unstaged_want)) {
             $unstaged_got = diff('HEAD');
         }
@@ -144,6 +147,16 @@ sub test_autofixup {
         $failed = 1;
     }
 
+    if (!defined($staged_want) && $staged_got) {
+        diag("staged_got=<<EOF\n${staged_got}EOF\nno staged changes expected\n");
+        $failed = 1;
+    }
+    if (defined($staged_want) && $staged_want ne $staged_got) {
+        diag("staged_got=<<EOF\n${staged_got}EOF\nstaged_want=<<EOF\n${staged_want}EOF\n");
+        $failed = 1;
+    }
+
+
     if (defined($unstaged_want) && $unstaged_want ne $unstaged_got) {
         diag("unstaged_got=<<EOF\n${unstaged_got}EOF\nunstaged_want=<<EOF\n${unstaged_want}EOF\n");
         $failed = 1;
@@ -752,5 +765,14 @@ index ae93045..16f9ec0 100644
 @@ -1 +1 @@
 -c1
 +c2
+EOF
+    , staged_want => <<'EOF'
+diff --git a/b b/b
+index c9c6af7..e6bfff5 100644
+--- a/b
++++ b/b
+@@ -1 +1 @@
+-b1
++b2
 EOF
 });