Update cheatsheets

gitk

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# gitk
+
+> A graphical Git repository browser.
+> More information: <https://git-scm.com/docs/gitk>.
+
+- Show the repository browser for the current Git repository:
+
+`gitk`
+
+- Show repository browser for a specific file or directory:
+
+`gitk {{path/to/file_or_directory}}`
+
+- Show commits made since 1 week ago:
+
+`gitk --since="{{1 week ago}}"`
+
+- Show commits older than 1/1/2016:
+
+`gitk --until="{{1/1/2015}}"`
+
+- Show at most 100 changes in all branches:
+
+` gitk --max-count={{100}} --all`