[v12.0/forgejo] fix: do better parsing of file modes (#9171)

**Backport: forgejo/forgejo#9161**

- No longer hardcode the file modes we expect, parse them as numbers and
do bitmask tricks that Git does so we allow a more variety of _weird_
file modes that can happen in the wild.
- Ref: https://codeberg.org/forgejo/forgejo/pulls/8900#issuecomment-6429175
- Resolves Codeberg/Community#2111

(cherry picked from commit 1cfd5e0b98)

Conflict resolution: trivial, choose the new code. Confliction arised
because v12 doesn't contain forgejo/forgejo#8900

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9171
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/git/parse_test.go

@@ -101,3 +101,38 @@ func TestParseTreeEntriesInvalid(t *testing.T) {
 	require.Error(t, err)
 	assert.Empty(t, entries)
 }
+
+func TestParseMode(t *testing.T) {
+	ok := func(t *testing.T, mode string, entry EntryMode) {
+		t.Helper()
+		actualEntry, err := parseMode(mode)
+		require.NoError(t, err)
+		assert.Equal(t, entry, actualEntry)
+	}
+
+	fail := func(t *testing.T, mode string) {
+		t.Helper()
+		entry, err := parseMode(mode)
+		require.Error(t, err)
+		assert.Zero(t, entry)
+	}
+
+	ok(t, "100644", EntryModeBlob)
+	ok(t, "100755", EntryModeExec)
+	ok(t, "100754", EntryModeExec)
+	ok(t, "100700", EntryModeExec)
+	ok(t, "100744", EntryModeExec)
+	ok(t, "120000", EntryModeSymlink)
+	ok(t, "120644", EntryModeSymlink)
+	ok(t, "160000", EntryModeCommit)
+	ok(t, "160644", EntryModeCommit)
+	ok(t, "040000", EntryModeTree)
+	ok(t, "040755", EntryModeTree)
+	ok(t, "040775", EntryModeTree)
+	ok(t, "040754", EntryModeTree)
+
+	fail(t, "not-a-number")
+	fail(t, "000000")
+	fail(t, "400000")
+	fail(t, "111111")
+}