worktree: add ability to create worktree with pre-existing branch

Currently, we always create a new branch after the new worktree's name
when creating a worktree. In some workflows, though, the caller may want
to check out an already existing reference instead of creating a new
one, which is impossible to do right now.

Add a new option `ref` to the options structure for adding worktrees. In
case it is set, a branch and not already checked out by another
worktree, we will re-use this reference instead of creating a new one.

1 files changed, 2 insertions, 1 deletions

diff --git a/include/git2/worktree.h b/include/git2/worktree.h
index d3fa88e3f..7aed93777 100644
--- a/include/git2/worktree.h
+++ b/include/git2/worktree.h
@@ -78,10 +78,11 @@ typedef struct git_worktree_add_options {
 	unsigned int version;
 
 	int lock; /**< lock newly created worktree */
+	git_reference *ref; /**< reference to use for the new worktree HEAD */
 } git_worktree_add_options;
 
 #define GIT_WORKTREE_ADD_OPTIONS_VERSION 1
-#define GIT_WORKTREE_ADD_OPTIONS_INIT {GIT_WORKTREE_ADD_OPTIONS_VERSION,0}
+#define GIT_WORKTREE_ADD_OPTIONS_INIT {GIT_WORKTREE_ADD_OPTIONS_VERSION,0,NULL}
 
 /**
  * Initializes a `git_worktree_add_options` with default vaules.