[Search git status] Preview only changes since file renamed (#263)

Previously, the git diff preview for a renamed path shows the entire file as being added. A more useful diff is to view only the modifications made to the file content. To accomplish this, we need a special condition for renames to diff the current (post-rename) path with the original path.

In addition, this PR fixes git status previews for renamed paths, which was accidentally broken in 38896f2.

Author: PatrickF1

README.md

@@ -135,7 +135,7 @@ They are always appended last to fzf's argument list. Because fzf uses the optio
 
 ### Change the commands used to preview directories and regular files
 
-The search directory feature, by default, calls `ls` to preview directories and `bat` to preview [regular files](https://stackoverflow.com/questions/6858452/what-is-a-regular-file-on-unix).
+The search directory feature, by default, calls `ls` to preview directories and `bat` to preview [regular files](https://stackoverflow.com/questions/6858452).
 
 To change the directory preview command (e.g. to use one of the many `ls` replacements such as `exa`), set the command in the `fzf_preview_dir_cmd` variable:
 

functions/_fzf_preview_changed_file.fish

@@ -2,7 +2,7 @@
 # arg should be a line from git status --short, e.g.
 # MM functions/_fzf_preview_changed_file.fish
 #  D README.md
-# R  LICENSE.md -> LICENSE
+# R  LICENSE -> "New License"
 function _fzf_preview_changed_file --argument-names path_status --description "Show the git diff of the given file."
     # remove quotes because they'll be interpreted literally by git diff
     # no need to requote when referencing $path because fish does not perform word splitting
@@ -12,8 +12,8 @@ function _fzf_preview_changed_file --argument-names path_status --description "S
     # https://git-scm.com/docs/git-status/2.35.0#_short_format
     set -l index_status (string sub --length 1 $path_status)
     set -l working_tree_status (string sub --start 2 --length 1 $path_status)
-    # no-prefix because the file is always being compared to itself so is unecessary
-    set diff_opts --color=always --no-prefix
+
+    set diff_opts --color=always
 
     if test $index_status = '?'
         _fzf_report_diff_type Untracked
@@ -27,7 +27,18 @@ function _fzf_preview_changed_file --argument-names path_status --description "S
     else
         if test $index_status != ' '
             _fzf_report_diff_type Staged
-            git diff --staged $diff_opts -- $path
+
+            # renames are only detected in the index, never working tree, so only need to test for it here
+            # https://stackoverflow.com/questions/73954214
+            if test $index_status = R
+                # diff the post-rename path with the original path, otherwise the diff will show the entire file as being added
+                set orig_and_new_path (string split --max 1 -- ' -> ' $path)
+                git diff --staged $diff_opts -- $orig_and_new_path[1] $orig_and_new_path[2]
+                # path currently has the form of "original -> current", so we need to correct it before it's used below
+                set path $orig_and_new_path[2]
+            else
+                git diff --staged $diff_opts -- $path
+            end
         end
 
         if test $working_tree_status != ' '

tests/_resources/alphabet 26 lines.txt

@@ -0,0 +1,26 @@
+aaaaaaaaaaa
+bbbbbbbbbbb
+ccccccccccc
+ddddddddddd
+eeeeeeeeeee
+fffffffffff
+ggggggggggg
+hhhhhhhhhhh
+iiiiiiiiiii
+jjjjjjjjjjj
+kkkkkkkkkkk
+lllllllllll
+mmmmmmmmmmm
+nnnnnnnnnnn
+ooooooooooo
+ppppppppppp
+qqqqqqqqqqq
+rrrrrrrrrrr
+sssssssssss
+ttttttttttt
+uuuuuuuuuuu
+vvvvvvvvvvv
+wwwwwwwwwww
+xxxxxxxxxxx
+yyyyyyyyyyy
+zzzzzzzzzzz

tests/preview_changed_file/path_with_spaces.fish renamed to tests/preview_changed_file/modified_path_with_spaces.fish

@@ -4,6 +4,6 @@ echo $expected_diff >>$path_with_space
 set output (_fzf_preview_changed_file " M \"$path_with_space\"")
 
 string match --entire --quiet $expected_diff $output
-@test "git diff successfully invoked on path with space" $status -eq 0
+@test "successfully previews modified path with spaces" $status -eq 0
 
 git restore $path_with_space

tests/preview_changed_file/renamed_in_index.fish

@@ -0,0 +1,17 @@
+set orig_path "tests/_resources/alphabet 26 lines.txt"
+set renamed_path "tests/_resources/alphabet lines.txt"
+set added_line a-very-unique-line
+git mv $orig_path $renamed_path
+echo $added_line >>$renamed_path
+git add $renamed_path
+
+set output (_fzf_preview_changed_file "R  \"$orig_path\" -> \"$renamed_path\"")
+
+# test that the added line shows up in the diff but not the entire file
+# "aaaaaaaaaa" is the first line in the file
+string match --entire --quiet $added_line $output &&
+    not string match --entire aaaaaaaaaa $output
+@test "shows only the modifications made to renamed file" $status -eq 0
+
+git mv $renamed_path $orig_path
+git restore --staged --worktree $orig_path