[PATCH v5] git gui: add directly calling merge tool from configuration

From: Tobias Boesch <tobias.boesch@miele.com>

git gui can open a merge tool when conflicts are detected (Right click in the diff of the file with conflicts). The merge tools that are allowed to use are hard coded into git gui.

If one wants to add a new merge tool it has to be added to git gui through a source code change. This is not convenient in comparison to how it works in git (without gui).

git itself has configuration options for a merge tools path and command in the git configuration. New merge tools can be set up there without a source code change.

Those options are used only by pure git in contrast to git gui. git calls the configured merge tools directly from the configuration while git Gui doesn't.

With this change git gui can call merge tools configured in the configuration directly without a change in git gui source code. It needs a configured "merge.tool" and a configured "mergetool.<mergetool name>.cmd" configuration entry as shown in the git-config manual page.

Configuration example:
[merge]
	tool = vscode
[mergetool "vscode"]
	path = the/path/to/Code.exe
	cmd = \"Code.exe\" --wait --merge \"$LOCAL\" \"$REMOTE\" \"$BASE\" \"$MERGED\"

Without the "mergetool.cmd" configuration and an unsupported "merge.tool" entry, git gui behaves mainly as before this change and informs the user about an unsupported merge tool. In addtition it also shows a hint to add a configuration entry to use the tool as an unsupported tool with degraded support.

If a wrong "mergetool.cmd" is configured by accident, it gets handled by git gui already. In this case git gui informs the user that the merge tool couldn't be opened. This behavior is preserved by this change and should not change.

"Beyond Compare 3" and "Visual Studio Code" were tested as manually configured merge tools.

Signed-off-by: Tobias Boesch <tobias.boesch@miele.com>
---
    git gui: add directly calling merge tool from gitconfig
    
    cc: Johannes Sixt j6t@kdbg.org
    
    Changes since v1:
    
     * Used existing option mergetool.cmd in gitconfig to trigger the direct
       call of the merge tool configured in the config instead adding a new
       option mergeToolFromConfig
     * Removed assignment of merge tool path to a variable and reused the
       already existing one: merget_tool_path
     * Changed formatting of the commit message
     * Added more context and an examples to the commit message
    
    Changes since v2:
    
     * Changed commit ident
     * Added hint to add a mergetool as an unsupprted tool
     * Minor typos
     * Highlighted proper nouns in commit message
     * Only using mergetool.cmd now - not using mergetool.path anymore
     * Removed gitconfig term in user message
     * Changed lines length of commit message
     * tcl commands in mergetool.cmd are now detected and not executed
       anymore
     * mergetool.cmd string parts are now substituted as list, not as a
       whole string
     * Made a more clear user hint on how to configure an unsupported
       mergetool
    
    Changes since v3:
    
     * Corrected wrong configuration option in commit message
     * Replaced "config" and "gitconfig" with "configuration" in commit
       message
     * Initialised cmdline list before appending values in loop
     * Removed hint that unsupported tools have degraded support
     * Changed popup message formatting in the popup and in the source code
    
    Changes since v4:
    
     * Removed trailing whitespace

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1773%2FToBoMi%2Fadd_merge_tool_from_config_file-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1773/ToBoMi/add_merge_tool_from_config_file-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/1773

Range-diff vs v4:

 1:  b5db8997b76 ! 1:  c37db60c218 git gui: add directly calling merge tool from configuration
     @@ git-gui/lib/mergetool.tcl: proc merge_resolve_tool2 {} {
      +			}
      +		} else {
      +			error_popup [mc "Unsupported merge tool '%s'.
     -+							
     ++
      +To use this tool, configure \"mergetool.%s.cmd\" as shown in the git-config\
      +manual page." $tool $tool]
      +			return

 git-gui/lib/mergetool.tcl | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)