Optimise ExtractsRef when there is no slash

ExtractsRef attempts to take a string containing slashes and figure out
which part's a reference to a tree-ish object, and which part's a path
in that tree-ish. For instance, `foo/bar/baz` might mean:

1. A ref of `foo`, a path of `bar/baz`
2. A ref of `foo/bar`, a path of `baz`
3. A ref of `foo/bar/baz`, and no path (the root)

To do this, it needs to load the full list of references (branches and
tags) for the repository, which can be slow and put pressure on Redis as
it uses SMEMBERS, which is an O(n) operation.

When there is no slash in the string, there is no ambiguity: it must be
a reference and no path (the third case above). In that case we can skip
loading the references entirely and just detect that by looking at the
string in question.

changelogs/unreleased/optimise-extractsref-when-there-is-no-path.yml

+---
+title: Remove some unnecessary Redis calls on commit lists
+merge_request: 38343
+author:
+type: performance

lib/extracts_ref.rb

@@ -47,6 +47,10 @@ module ExtractsRef
     if id =~ /^(\h{40})(.+)/
       # If the ref appears to be a SHA, we're done, just split the string
       pair = $~.captures
+    elsif id.exclude?('/')
+      # If the ID contains no slash, we must have a ref and no path, so
+      # we can skip the Redis calls below
+      pair = [id, '']
     else
       # Otherwise, attempt to detect the ref using a list of the repository_container's
       # branches and tags
@@ -71,12 +75,10 @@ module ExtractsRef
       end
     end
 
-    pair[0] = pair[0].strip
-
-    # Remove ending slashes from path
-    pair[1].gsub!(%r{^/|/$}, '')
-
-    pair
+    [
+      pair[0].strip,
+      pair[1].gsub(%r{^/|/$}, '') # Remove leading and trailing slashes from path
+    ]
   end
 
   # Assigns common instance variables for views working with Git tree-ish objects

spec/support/shared_examples/path_extraction_shared_examples.rb

@@ -88,9 +88,16 @@ RSpec.shared_examples 'extracts refs' do
         expect(extract_ref('stable')).to eq(['stable', ''])
       end
 
-      it 'extracts the longest matching ref' do
-        expect(extract_ref('release/app/v1.0.0/README.md')).to eq(
-          ['release/app/v1.0.0', 'README.md'])
+      it 'does not fetch ref names when there is no slash' do
+        expect(self).not_to receive(:ref_names)
+
+        extract_ref('master')
+      end
+
+      it 'fetches ref names when there is a slash' do
+        expect(self).to receive(:ref_names).and_call_original
+
+        extract_ref('release/app/v1.0.0')
       end
     end
 
@@ -113,6 +120,11 @@ RSpec.shared_examples 'extracts refs' do
       it 'falls back to a primitive split for an invalid ref' do
         expect(extract_ref('stable/CHANGELOG')).to eq(%w(stable CHANGELOG))
       end
+
+      it 'extracts the longest matching ref' do
+        expect(extract_ref('release/app/v1.0.0/README.md')).to eq(
+          ['release/app/v1.0.0', 'README.md'])
+      end
     end
   end
 end