[PATCH v2] git-difftool-helper.sh: learn a new way go back to last save point

From: ZheNing Hu <adlternative@gmail.com>

`git difftool` only allow us to select file to view in turn. If there is a commit with many files and we exit in the search, We will have to traverse list again to get the file diff which we want to see. Therefore, here is a new method: every time before we view the file diff, the current coordinates will be stored in `GIT_DIR/difftool-last-position`, this file will be deleted after successful traversing. But if an unexpected exit occurred midway or users similar to using "ctrl+c" kill the process,and the user wants to redo the same `git difftoool`, git will view the coordinates in the save point, ask user if they want continue from the last position. This will improve the user experience.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
---
    git-difftool-helper.sh: learn a new way skip to save point
    
    git user may should travel the diff list to choice file diff to view, if
    they exit in midway,they must travel it again. By saving current
    coordinates in GIT_DIR/difftool-last-position method, provides a
    possibility for this user-friendly solution.
    
    this patch's origin discuss is here:
    https://lore.kernel.org/git/gOXOaoqn-E9A2ob7ykWEcDc7ZxmSwAjcP5CCFKfr5ejCOWZQ1lfAUZcbgYT9AyQCcDgJvCrnrtziXiels-Hxol3xlkGTVHk24SvAdaSUtKQ=@rtzoeller.com/
    
    Thanks!

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-870%2Fadlternative%2Fdifftool_save_point-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-870/adlternative/difftool_save_point-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/870

Range-diff vs v1:

 1:  e77c3e33ba85 ! 1:  2468eaff322b git-difftool-helper.sh: learn a new way skip to save point
     @@ Metadata
      Author: ZheNing Hu <adlternative@gmail.com>
      
       ## Commit message ##
     -    git-difftool-helper.sh: learn a new way skip to save point
     +    git-difftool-helper.sh: learn a new way go back to last save point
      
     -    `git difftool` only allow us to select file to view In turn.
     -    If there is a commit with many files and we exit in search,
     +    `git difftool` only allow us to select file to view in turn.
     +    If there is a commit with many files and we exit in the search,
          We will have to traverse list again to get the file diff which
     -    we want to see.Therefore,here is a new method:every time before
     -    we view the file diff,the current coordinates will be stored in
     -    `GIT_DIR/difftool_skip_to`,this file will be deleted after
     -    successful traversing.But if an unexpected exit occurred midway,
     -    git will view the coordinates in the save point,ask user if they
     -    want continue from the last saved point.This will improve the
     -    user experience.
     +    we want to see. Therefore, here is a new method: every time before
     +    we view the file diff, the current coordinates will be stored in
     +    `GIT_DIR/difftool-last-position`, this file will be deleted after
     +    successful traversing. But if an unexpected exit occurred midway or
     +    users similar to using "ctrl+c" kill the process,and the user wants
     +    to redo the same `git difftoool`, git will view the coordinates in
     +    the save point, ask user if they want continue from the last position.
     +    This will improve the user experience.
      
          Signed-off-by: ZheNing Hu <adlternative@gmail.com>
      
     @@ git-difftool--helper.sh
       # Copyright (c) 2009, 2010 David Aguilar
       
       TOOL_MODE=diff
     -+GIT_DIFFTOOL_SKIP_TO_FILE="$GIT_DIR/difftool-skip-to"
     ++GIT_DIFFTOOL_LAST_POSITION="$GIT_DIR/difftool-last-position"
     ++DIFFTOOL_FIRST_NUM="1"
       . git-mergetool--lib
       
       # difftool.prompt controls the default prompt/no-prompt behavior
     @@ git-difftool--helper.sh: launch_merge_tool () {
       	# the user with the real $MERGED name before launching $merge_tool.
       	if should_prompt
       	then
     -+		if test -f "$GIT_DIFFTOOL_SKIP_TO_FILE"
     ++		if test -f "$GIT_DIFFTOOL_LAST_POSITION"
      +		then
     -+			SAVE_POINT_NUM=$(cat "$GIT_DIFFTOOL_SKIP_TO_FILE")
     -+			if test $SAVE_POINT_NUM -le $GIT_DIFF_PATH_TOTAL &&
     -+				test $SAVE_POINT_NUM -gt $GIT_DIFF_PATH_COUNTER
     ++			if SAVE_POINT_NUM=$(cat 2>/dev/null "$GIT_DIFFTOOL_LAST_POSITION") &&
     ++				test "$SAVE_POINT_NUM" -le "$GIT_DIFF_PATH_TOTAL" &&
     ++					test "$SAVE_POINT_NUM" -gt "$GIT_DIFF_PATH_COUNTER"
      +			then
     -+				# choice skip or not skip when check first file.
     -+				if test $GIT_DIFF_PATH_COUNTER -eq "1"
     ++				if test "$GIT_DIFF_PATH_COUNTER" -eq "$DIFFTOOL_FIRST_NUM"
      +				then
     -+					printf "do you want to skip to last time difftool save point($SAVE_POINT_NUM) [Y/n]?"
     ++					printf "Do you want to start from the possible last file you were viewing? [Y/n]?"
      +					read skip_ans || return
      +					if test "$skip_ans" = y
      +					then
     @@ git-difftool--helper.sh: launch_merge_tool () {
      +				fi
      +			fi
      +		fi
     -+		# write the current coordinates to .git/difftool-skip-to
     -+		if test !$SAVE_POINT_NUM || $SAVE_POINT_NUM -ne $GIT_DIFF_PATH_COUNTER
     ++		if test -z "$SAVE_POINT_NUM" ||
     ++			test "$SAVE_POINT_NUM" -ne "$GIT_DIFF_PATH_COUNTER"
      +		then
     -+			echo $GIT_DIFF_PATH_COUNTER > $GIT_DIFFTOOL_SKIP_TO_FILE
     ++			echo "$GIT_DIFF_PATH_COUNTER" >"$GIT_DIFFTOOL_LAST_POSITION"
      +		fi
       		printf "\nViewing (%s/%s): '%s'\n" "$GIT_DIFF_PATH_COUNTER" \
       			"$GIT_DIFF_PATH_TOTAL" "$MERGED"
     @@ git-difftool--helper.sh: else
       	done
       fi
       
     -+if test -f $GIT_DIFFTOOL_SKIP_TO_FILE &&
     -+	test $GIT_DIFF_PATH_COUNTER -eq $GIT_DIFF_PATH_TOTAL
     ++if test "$GIT_DIFF_PATH_COUNTER" -eq "$GIT_DIFF_PATH_TOTAL"
      +then
     -+	rm $GIT_DIFFTOOL_SKIP_TO_FILE
     ++	rm -f "$GIT_DIFFTOOL_LAST_POSITION"
      +
      +fi
       exit 0

 git-difftool--helper.sh | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)