Made git_stash only stash untracked files when capable

Not all versions of git support inclusion of untracked files when stashing. This
change probes git to determine if it is capable, only using the option when it's
available.

Ticket: CFE-3383
Changelog: Title

Author: nickanderson

lib/vcs.cf

@@ -156,7 +156,7 @@ bundle agent git_clean(repo_path)
 }
 
 bundle agent git_stash(repo_path, stash_name)
-# @brief Stash any changes (including untracked files) in repo_path
+# @brief Stash any changes (including untracked files if git is capable) in repo_path
 # @param repo_path Path to the clone
 # @param stash_name Stash name
 #
@@ -169,14 +169,31 @@ bundle agent git_stash(repo_path, stash_name)
 #      comment => "Stash any changes, including untracked files";
 # ```
 {
+  classes:
+    _stdlib_path_exists_git::
+      "_git_stash_supports_including_untracked_files" -> { "CFE-3383" }
+        expression => regcmp( ".*--include-untracked.*",
+                              execresult( "$(paths.git) stash --help", noshell ) );
+
+  vars:
+      "_stash_options"
+        string => concat( "save ",
+                          "--quiet ",
+                          ifelse( "_git_stash_supports_including_untracked_files",
+                                  "--include-untracked", ""),
+                          "$(stash_name)");
+
   methods:
-      "" usebundle => git($(repo_path), "stash", 'save --quiet --include-untracked "$(stash_name)"'),
+      "" usebundle => git($(repo_path), "stash", $(_stash_options)),
       comment => "So that we don't lose any trail of what happened and so that
                     we don't accidentally delete something important we stash any
                     changes.
   Note:
                       1. This promise will fail if user.email is not set
                       2. We are respecting ignored files.";
+
+    !_stdlib_path_exists_git::
+      "Warning: bundle '$(this.bundle)' actuated, but git not found";
 }
 
 bundle agent git_stash_and_clean(repo_path)