git-completion fixes (#1213)

- update push logic to support pushing a :delete

Also used for-each-ref as a replacement for `branch --no-color`

I've double-checked the behavior and found that the refactor actually
fixes 2 bugs in the original implementation that I missed initially:

- Symrefs: git branch output included -> target (e.g. origin/HEAD ->
origin/main), which created invalid completions. for-each-ref gives
clean origin/HEAD.
- Detached HEAD: git branch output included (HEAD detached at ...),
which was garbage. for-each-ref ignores it. I've updated the walkthrough
with these details.

Author: kevmoo

custom-completions/git/git-completions.nu

@@ -98,7 +98,7 @@ module git-completion-utils {
 }
 
 def "nu-complete git available upstream" [] {
-  ^git branch --no-color -a | lines | each { |line| $line | str replace '* ' "" | str trim }
+  ^git for-each-ref --format '%(refname:short)' refs/heads refs/remotes | lines
 }
 
 def "nu-complete git remotes" [] {
@@ -124,12 +124,12 @@ def "nu-complete git commits current branch" [] {
 
 # Yield local branches like `main`, `feature/typo_fix`
 def "nu-complete git local branches" [] {
-  ^git branch --no-color | lines | each { |line| $line | str replace '* ' "" | str replace '+ ' ""  | str trim }
+  ^git for-each-ref --format '%(refname:short)' refs/heads | lines
 }
 
 # Yield remote branches like `origin/main`, `upstream/feature-a`
 def "nu-complete git remote branches with prefix" [] {
-  ^git branch --no-color -r | lines | parse -r '^\*?(\s*|\s*\S* -> )(?P<branch>\S*$)' | get branch | uniq
+  ^git for-each-ref --format='%(refname:lstrip=2)' refs/remotes | lines
 }
 
 # Yield local and remote branch names which can be passed to `git merge`
@@ -382,10 +382,43 @@ export extern "git fetch" [
   -6                                            # Use IPv6 addresses, ignore IPv4 addresses
 ]
 
+# Yield local branches and (if remote is specified) remote branches with colon prefix
+def "nu-complete git push" [context: string, position: int] {
+  use git-completion-utils *
+  let preceding = $context | str substring ..$position
+  let tokens = $preceding | str trim | args-split | where ($it not-in $GIT_SKIPABLE_FLAGS)
+
+  # Check if we have a remote argument (2nd token, 1st is 'git', 2nd is 'push', 3rd is remote)
+  # BUT, args-split might be different depending on how it's called.
+  # "git push origin" -> ["git", "push", "origin"]
+  # If we have at least 3 tokens, the 3rd one IS likely the remote.
+  # We should double check if the 3rd token is actually a remote.
+  
+  mut remote = ""
+  if ($tokens | length) >= 3 {
+    $remote = $tokens.2
+  }
+
+  let local_branches = (nu-complete git local branches)
+  
+  if ($remote | is-empty) {
+    return $local_branches
+  }
+
+  # If we have a remote, find branches for that remote
+  # Use plumbing command to get remote branches, excluding HEAD
+  let remote_branches = (^git for-each-ref --format='%(refname:lstrip=3)' $'refs/remotes/($remote)' | lines | where $it != 'HEAD')
+  
+  # Prefix them with :
+  let deletion_candidates = ($remote_branches | each { |it| $":($it)" })
+
+  $local_branches | append $deletion_candidates
+}
+
 # Push changes
 export extern "git push" [
   remote?: string@"nu-complete git remotes",         # the name of the remote
-  ...refs: string@"nu-complete git local branches"   # the branch / refspec
+  ...refs: string@"nu-complete git push"             # the branch / refspec
   --all                                              # push all refs
   --atomic                                           # request atomic transaction on remote side
   --delete(-d)                                       # delete refs