rust: simplify pattern file parsing Previously we kept the pattern syntax as a &[u8] until the last possible moment, which meant we had to handle potential errors when parsing that &[u8]. But such errors could never actually occur, given the structure of the code. Now we parse it eagerly (in two places) and pass it around as a PatternSyntax, so we can delete some error handling code. parse_one_pattern is in particular useful for parsing patterns passed on the command line, as we'll support later in this series.

--- a/rust/hg-core/src/narrow.rs	Wed Aug 02 09:57:29 2023 -0400
+++ b/rust/hg-core/src/narrow.rs	Wed Aug 09 18:08:28 2023 -0400
@@ -74,6 +74,7 @@
         Path::new(""),
         None,
         false,
+        true,
     )?;
     warnings.extend(subwarnings.into_iter().map(From::from));
 
@@ -85,6 +86,7 @@
         Path::new(""),
         None,
         false,
+        true,
     )?;
     if !patterns.is_empty() {
         warnings.extend(subwarnings.into_iter().map(From::from));