git-gui: add page

pages/common/git-gui.md

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+# git gui
+
+> A GUI for Git to manage branches, commits, and remotes, and perform local merges.
+> See also: `git-cola`, `gitk`.
+> More information: <https://git-scm.com/docs/git-gui>.
+
+- Launch the GUI:
+
+`git gui`
+
+- Show a specific file with author name and commit hash on each line:
+
+`git gui blame {{path/to/file}}`
+
+- Open `git gui blame` in a specific revision:
+
+`git gui blame {{revision}} {{path/to/file}}`
+
+- Open `git gui blame` and scroll the view to center on a specific line:
+
+`git gui blame --line={{line}} {{path/to/file}}`
+
+- Open a window to make one commit and return to the shell when it is complete:
+
+`git gui citool`
+
+- Open `git gui citool` in the "Amend Last Commit" mode:
+
+`git gui citool --amend`
+
+- Open `git gui citool` in a read-only mode:
+
+`git gui citool --nocommit`
+
+- Show a browser for the tree of a specific branch, opening the blame tool when clicking on the files:
+
+`git gui browser maint`