fix(git): optimize fetch to selectively include tags and checkout exact refs

- Disabled unconditional addition of "remote.origin.tagOpt" config; only add if not fetching exact ref
- Introduced exactRef flag to detect when fetching a specific origin ref
- When fetching exact ref, checkout uses "FETCH_HEAD" instead of the ref name to avoid detached head advice
- Cleaned up commented-out code and improved fetch argument logic accordingly

Signed-off-by: Filip Pytloun <filip@pytloun.cz>

Author: fpytloun

pkg/vendir/fetch/git/git.go

@@ -166,17 +166,20 @@ func (t *Git) fetch(dstPath string, tempArea ctlfetch.TempArea, bundle string) e
 		}
 	}
 
-	argss = append(argss, []string{"config", "remote.origin.tagOpt", "--tags"})
-
 	if bundle != "" {
 		argss = append(argss, []string{"bundle", "unbundle", bundle})
 	}
 
+	exactRef := false
 	{
 		fetchArgs := []string{"fetch", "origin"}
 		if strings.HasPrefix(t.opts.Ref, "origin/") {
 			// only fetch the exact ref we're seeking
 			fetchArgs = append(fetchArgs, t.opts.Ref[7:])
+			exactRef = true
+		} else {
+			// Only fetch tags if not fetching exact ref
+			argss = append(argss, []string{"config", "remote.origin.tagOpt", "--tags"})
 		}
 		if t.opts.Depth > 0 {
 			fetchArgs = append(fetchArgs, "--depth", strconv.Itoa(t.opts.Depth))
@@ -201,7 +204,12 @@ func (t *Git) fetch(dstPath string, tempArea ctlfetch.TempArea, bundle string) e
 		}
 	}
 
-	_, _, err = t.cmdRunner.Run([]string{"-c", "advice.detachedHead=false", "checkout", ref}, env, dstPath)
+	checkoutRef := ref
+	if exactRef {
+		checkoutRef = "FETCH_HEAD"
+	}
+
+	_, _, err = t.cmdRunner.Run([]string{"-c", "advice.detachedHead=false", "checkout", checkoutRef}, env, dstPath)
 	if err != nil {
 		return err
 	}