git gui: add directly calling merge tool from configuration

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"]
	cmd = \"the/path/to/Code.exe\" --wait --merge \"$LOCAL\" \"$REMOTE\" \"$BASE\" \"$MERGED\"

Without the "mergetool.<mergetool name>.cmd" entry 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.<mergetool name>.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>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

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