refactor: Make commit.gpgSign match official capitalization

The actual usage is case insensitive, so this doesn't actually matter.
But if fills my heart with joy.

The test is case sensitive, but the actual response to `git config
commit.gpgSign` is equivalent to `git config commit.gppsign`

pkg/commands/git_commands/rebase_test.go

@@ -128,7 +128,7 @@ func TestRebaseDiscardOldFileChanges(t *testing.T) {
 		},
 		{
 			testName:               "returns error when using gpg",
-			gitConfigMockResponses: map[string]string{"commit.gpgsign": "true"},
+			gitConfigMockResponses: map[string]string{"commit.gpgSign": "true"},
 			commits:                []*models.Commit{{Name: "commit", Hash: "123456"}},
 			commitIndex:            0,
 			fileName:               []string{"test999.txt"},