Metadata fetching: Handle ENOENT when git not installed (#6409)

This git fallback is being hit whenever there's a non-`200` response
code. However, if `git` isn't installed, this will result in throwing:
```shell
Errno::ENOENT with No such file or directory - git
```

So catch the error and handle it.

common/lib/dependabot/git_metadata_fetcher.rb

@@ -112,13 +112,17 @@ def fetch_raw_upload_pack_with_git_for(uri)
       command = "git ls-remote #{service_pack_uri}"
       command = SharedHelpers.escape_command(command)
 
-      stdout, stderr, process = Open3.capture3(env, command)
-      # package the command response like a HTTP response so error handling
-      # remains unchanged
-      if process.success?
-        OpenStruct.new(body: stdout, status: 200)
+      begin
+        stdout, stderr, process = Open3.capture3(env, command)
+        # package the command response like a HTTP response so error handling remains unchanged
+      rescue Errno::ENOENT => e # thrown when `git` isn't installed...
+        OpenStruct.new(body: e.message, status: 500)
       else
-        OpenStruct.new(body: stderr, status: 500)
+        if process.success?
+          OpenStruct.new(body: stdout, status: 200)
+        else
+          OpenStruct.new(body: stderr, status: 500)
+        end
       end
     end