git-gui: accommodate for intent-to-add files

As of Git v2.28.0, the diff for files staged via `git add -N` marks them
as new files. Git GUI was ill-prepared for that, and this patch teaches
Git GUI about them.

Please note that this will not even fix things with v2.28.0, as the
`rp/apply-cached-with-i-t-a` patches are required on Git's side, too.

This fixes #2779

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

Author: dscho

git-gui/git-gui.sh

@@ -2078,6 +2078,7 @@ set all_icons(U$ui_index)   file_merge
 set all_icons(T$ui_index)   file_statechange
 
 set all_icons(_$ui_workdir) file_plain
+set all_icons(A$ui_workdir) file_plain
 set all_icons(M$ui_workdir) file_mod
 set all_icons(D$ui_workdir) file_question
 set all_icons(U$ui_workdir) file_merge
@@ -2104,6 +2105,7 @@ foreach i {
 		{A_ {mc "Staged for commit"}}
 		{AM {mc "Portions staged for commit"}}
 		{AD {mc "Staged for commit, missing"}}
+		{AA {mc "Intended to be added"}}
 
 		{_D {mc "Missing"}}
 		{D_ {mc "Staged for removal"}}

git-gui/lib/diff.tcl

@@ -568,7 +568,8 @@ proc apply_or_revert_hunk {x y revert} {
 	if {$current_diff_side eq $ui_index} {
 		set failed_msg [mc "Failed to unstage selected hunk."]
 		lappend apply_cmd --reverse --cached
-		if {[string index $mi 0] ne {M}} {
+		set file_state [string index $mi 0]
+		if {$file_state ne {M} && $file_state ne {A}} {
 			unlock_index
 			return
 		}
@@ -581,7 +582,8 @@ proc apply_or_revert_hunk {x y revert} {
 			lappend apply_cmd --cached
 		}
 
-		if {[string index $mi 1] ne {M}} {
+		set file_state [string index $mi 1]
+		if {$file_state ne {M} && $file_state ne {A}} {
 			unlock_index
 			return
 		}
@@ -673,7 +675,8 @@ proc apply_or_revert_range_or_line {x y revert} {
 		set failed_msg [mc "Failed to unstage selected line."]
 		set to_context {+}
 		lappend apply_cmd --reverse --cached
-		if {[string index $mi 0] ne {M}} {
+		set file_state [string index $mi 0]
+		if {$file_state ne {M} && $file_state ne {A}} {
 			unlock_index
 			return
 		}
@@ -688,7 +691,8 @@ proc apply_or_revert_range_or_line {x y revert} {
 			lappend apply_cmd --cached
 		}
 
-		if {[string index $mi 1] ne {M}} {
+		set file_state [string index $mi 1]
+		if {$file_state ne {M} && $file_state ne {A}} {
 			unlock_index
 			return
 		}