feat: respect core.commentChar setting for escaping comment characters in commit messages

Bring StGit in parity with git's behavior for comment characters.  Setting
`core.commentChar` to a value other than '#' allows the use of certain markup
like markdown in commit messages.  Git for that reason allows overriding the
comment character to any string value.

This change incurs a small amount of extra allocations for constructing strings
at runtime taking into account the user's choice of comment character.

issue: 600

Author: NonLogicalDev

src/ext/repository.rs

@@ -3,7 +3,7 @@
 use std::borrow::Cow;
 
 use anyhow::{anyhow, Result};
-use bstr::BStr;
+use bstr::{BStr, ByteSlice};
 
 use crate::{
     stupid::Stupid,
@@ -78,6 +78,13 @@ pub(crate) trait RepositoryExtended {
 
     /// [`gix::Repository::rev_parse_single()`] with StGit-specific error mapping.
     fn rev_parse_single_ex(&self, spec: &str) -> Result<gix::Id<'_>>;
+
+    /// Get the comment symbol from git config.
+    ///
+    /// Returns the configured `core.commentChar` value, defaulting to "#" if not set.
+    /// Git's default is '#' and it also supports 'auto' which we treat as '#'.
+    /// Note: git allows this to be a string, not just a single character.
+    fn get_comment_symbol(&self) -> String;
 }
 
 /// Options for creating a git commit object.
@@ -330,4 +337,21 @@ impl RepositoryExtended for gix::Repository {
                 }
             })
     }
+
+    fn get_comment_symbol(&self) -> String {
+        let config = self.config_snapshot();
+        config
+            .string("core.commentChar")
+            .and_then(|value| value.to_str().ok().map(|s| s.to_string()))
+            .map(|value_str| {
+                // Git supports 'auto' which means it will pick a character that doesn't
+                // conflict with the message. We just use '#' for 'auto'.
+                if value_str == "auto" || value_str.is_empty() {
+                    "#".to_string()
+                } else {
+                    value_str
+                }
+            })
+            .unwrap_or("#".to_string())
+    }
 }