Only bump error count when we are sure that the diagnostic is not a repetition.

This ensures that if we emit the same diagnostic twice, the error count will
match the real number of errors shown to the user.

Fixes #42106

src/test/ui/feature-gate/issue-43106-gating-of-rustc_deprecated.stderr

@@ -42,5 +42,5 @@ error: stability attributes may not be used outside of the standard library
 35 |     #[rustc_deprecated = "1500"] impl S { }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: aborting due to 8 previous errors
 

src/test/ui/feature-gate/issue-43106-gating-of-stable.stderr

@@ -42,5 +42,5 @@ error: stability attributes may not be used outside of the standard library
 35 |     #[stable = "1300"] impl S { }
    |     ^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: aborting due to 8 previous errors
 

src/test/ui/feature-gate/issue-43106-gating-of-unstable.stderr

@@ -42,5 +42,5 @@ error: stability attributes may not be used outside of the standard library
 35 |     #[unstable = "1200"] impl S { }
    |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: aborting due to 8 previous errors
 

src/test/ui/issue-42106.stderr

@@ -8,5 +8,5 @@ error[E0502]: cannot borrow `*collection` as mutable because `collection` is als
 14 | }
    | - immutable borrow ends here
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

src/test/ui/span/macro-ty-params.stderr

@@ -22,5 +22,5 @@ error: generic arguments in macro path
 20 |     m!(MyTrait<>); //~ ERROR generic arguments in macro path
    |               ^^
 
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors