[PATCH] prefer "git COMMAND" over "git-COMMAND" in gitk

Preferring git _space_ COMMAND over git _dash_ COMMAND allows the
user to have only git and gitk in their path. e.g. when git and gitk
are symbolic links in a personal bin directory to the real git and gitk.

Signed-off-by: Paul Mackerras <paulus@samba.org>