lib: add type support for refspecs

Fully qualified git refs (e.g., `+refs/head/main:refs/remotes/origin/main`)
are commonly used throughout the git glue code.

It is laborious and error prone to keep parsing refspecs when we need
only a segment of it.

This commits adds a type, `RefSpec`, which does exactly that

lib/src/git.rs

@@ -79,6 +79,34 @@ impl fmt::Display for RefName {
     }
 }
 
+/// a refspec is formatted as <source>:<destination>
+/// if it is forced, a '+' is prepended
+#[derive(Debug, Hash, PartialEq, Eq)]
+pub(crate) struct RefSpec {
+    forced: bool,
+    source: String,
+    destination: String,
+}
+
+impl RefSpec {
+    fn forced(source: impl Into<String>, destination: impl Into<String>) -> Self {
+        RefSpec {
+            forced: true,
+            source: source.into(),
+            destination: destination.into(),
+        }
+    }
+
+    pub(crate) fn to_git_format(&self) -> String {
+        format!(
+            "{}{}:{}",
+            if self.forced { "+" } else { "" },
+            self.source,
+            self.destination
+        )
+    }
+}
+
 pub fn parse_git_ref(ref_name: &str) -> Option<RefName> {
     if let Some(branch_name) = ref_name.strip_prefix("refs/heads/") {
         // Git CLI says 'HEAD' is not a valid branch name
@@ -1452,8 +1480,14 @@ impl<'a> GitFetch<'a> {
                          * because `to_glob()` escapes such `*`s as `[*]`. */
                         |glob| !glob.contains(INVALID_REFSPEC_CHARS),
                     )
-                    .map(|glob| format!("+refs/heads/{glob}:refs/remotes/{remote_name}/{glob}"))
+                    .map(|glob| {
+                        RefSpec::forced(
+                            format!("refs/heads/{glob}"),
+                            format!("refs/remotes/{remote_name}/{glob}"),
+                        )
+                    })
             })
+            .map(|refspec| refspec.map(|r| r.to_git_format()))
             .collect::<Option<_>>()
             .ok_or(GitFetchError::InvalidBranchPattern)?;
         if refspecs.is_empty() {