GitLens should use diff.guitool if available

[pluma]

• GitLens Version: 5.7.1
• VSCode Version: 1.17.2
• OS Version: N/A

(Continued from #57)

Currently gitlens tries to use diff.tool for directory diffs and complains if it is not available.

Setting diff.tool to a GUI tool makes the git CLI awkward to use. However git allows explicitly using a GUI tool by invoking git diff --gui which looks for a git config called diff.guitool.

GitLens should try to use diff.guitool if it has been set and fall back to diff.tool otherwise. This means that if GitLens invokes git diff it should also pass the --gui flag as appropriate. GitLens should still warn about the git config if neither diff.guitool nor diff.tool have been set.

[pluma]

Ooh, nice. Thank you!

[github-actions]

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