32395: _git: diff: refactor and fix wrong completions

Before this, there were several cases where the completion
would offer the wrong things:

$ git diff branch -- <tab>

would try to complete "changed in workdir files", but needs to
complete all "tree files in HEAD".

$ git diff --cached -- file1 file2 <tab>
would try to complete "changed in workdir files" but needs to
complete "changed in index files".

...

After this change all possible combinations are taken into
account and completion should work properly.

1 files changed, 44 insertions, 33 deletions

diff --git a/Completion/Unix/Command/_git b/Completion/Unix/Command/_git
index 8562ab21a..e9905cce6 100644
--- a/Completion/Unix/Command/_git
+++ b/Completion/Unix/Command/_git
@@ -722,63 +722,74 @@ _git-diff () {
 
   case $state in
     (from-to-file)
+      # If "--" is part of $opt_args, this means it was specified before any
+      # $words arguments. This means that no heads are specified in front, so
+      # we need to complete *changed* files only.
+      if [[ -n ${opt_args[(I)--]} ]]; then
+        if [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
+          __git_changed-in-index_files && ret=0
+        else
+          __git_changed-in-working-tree_files && ret=0
+        fi
+        return ret
+      fi
+
+      # Otherwise, more complex conditions need to be checked.
       case $CURRENT in
         (1)
-          if [[ -n ${opt_args[(I)--]} ]]; then
-            if [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
-              __git_changed-in-index_files && ret=0
-            else
-              __git_changed-in-working-tree_files && ret=0
-            fi
-          else
-            local files_alt='files::__git_changed-in-working-tree_files'
-
-            if [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
-              files_alt='files::__git_changed-in-index_files'
-            fi
-
-            _alternative \
-              'commit-ranges::__git_commit_ranges' \
-              'blobs-and-trees-in-treeish::__git_blobs_and_trees_in_treeish' \
-              $files_alt \
-              'blobs::__git_blobs ' && ret=0
+          local files_alt='files::__git_changed-in-working-tree_files'
+          if [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
+            files_alt='files::__git_changed-in-index_files'
           fi
+
+          _alternative \
+            'commit-ranges::__git_commit_ranges' \
+            'blobs-and-trees-in-treeish::__git_blobs_and_trees_in_treeish' \
+            $files_alt \
+            'blobs::__git_blobs ' && ret=0
           ;;
         (2)
+          # Check if first argument is something special. In case of committish ranges and committishs offer a full list compatible completions.
           if __git_is_committish_range $line[1]; then
+            # Example: git diff branch1..branch2 <tab>
             __git_tree_files ${PREFIX:-.} $(__git_committish_range_last $line[1]) && ret=0
           elif __git_is_committish $line[1] || __git_is_treeish $line[1]; then
-            if [[ -n ${opt_args[(I)--]} ]]; then
-              __git_changed-in-working-tree_files && ret=0
-            else
-              _alternative \
-                'commits::__git_commits' \
-                'blobs-and-trees-in-treeish::__git_blobs_and_trees_in_treeish' \
-                'files::__git_changed-in-working-tree_files' && ret=0
-            fi
+            # Example: git diff branch1 <tab>
+            _alternative \
+              'commits::__git_commits' \
+              'blobs-and-trees-in-treeish::__git_blobs_and_trees_in_treeish' \
+              'files::__git_tree_files ${PREFIX:-.} HEAD' && ret=0
           elif __git_is_blob $line[1]; then
-            if [[ -n ${opt_args[(I)--]} ]]; then
-              __git_cached_files && ret=0
-            else
-              _alternative \
-                'files::__git_cached_files' \
-                'blobs::__git_blobs' && ret=0
-            fi
+            _alternative \
+              'files::__git_cached_files' \
+              'blobs::__git_blobs' && ret=0
           elif [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
+            # Example: git diff --cached file1 <tab>
             __git_changed-in-index_files && ret=0
           else
+            # Example: git diff file1 <tab>
             __git_changed-in-working-tree_files && ret=0
           fi
           ;;
         (*)
           if __git_is_committish_range $line[1]; then
+            # Example: git diff branch1..branch2 file1 <tab>
             __git_tree_files ${PREFIX:-.} $(__git_committish_range_last $line[1]) && ret=0
           elif { __git_is_committish $line[1] && __git_is_committish $line[2] } ||
               __git_is_treeish $line[2]; then
+            # Example: git diff branch1 branch2 <tab>
             __git_tree_files ${PREFIX:-.} $line[2] && ret=0
+          elif __git_is_committish $line[1] || __git_is_treeish $line[1]; then
+            # Example: git diff branch file1 <tab>
+            # Example: git diff branch -- f<tab>
+            __git_tree_files ${PREFIX:-.} HEAD && ret=0
           elif __git_is_blob $line[1] && __git_is_blob $line[2]; then
             _nothing
+          elif [[ -n ${opt_args[(I)--cached|--staged]} ]]; then
+            # Example: git diff --cached file1 file2 <tab>
+            __git_changed-in-index_files && ret=0
           else
+            # Example: git diff file1 file2 <tab>
             __git_changed-in-working-tree_files && ret=0
           fi
           ;;