gitk: replace tabs with spaces

gitk: replace tabs with spaces

The source code is a mix of tabs and spaces. The indentation style
currently is four spaces per indent level but uses tabs every other
level (at eight spaces). Fix this inconsistent spacing and tabbing by
just using a space-indent for everything.

This was done mechanically by running:

	$ expand -i gitk >gitk.new
	$ mv gitk.new gitk

This patch should be empty with `--ignore-all-space`.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Paul Mackerras <redacted>