gitk: Disable "Reset %s branch to here" when on a detached head

gitk: Disable "Reset %s branch to here" when on a detached head

When we are on a detached head - since gitk does not display where
we are - reset has no sense, so disable the relevant line on the
context menu, and point out to the user that we are on a detached head.

Otherwise, a reset from gitk when on a detached head returns the
error:

can't read "headids()": no such element in array
can't read "headids()": no such element in array
    while executing
"removehead $headids($name) $name"
    (procedure "movehead" line 4)
    invoked from within
"movehead $newhead $mainhead"
    (procedure "readresetstat" line 20)
    invoked from within
"readresetstat file4"
    ("eval" body line 1)
    invoked from within
"eval $script"
    (procedure "dorunq" line 9)
    invoked from within
"dorunq"
    ("after" script)

[paulus@samba.org: changed menu item to "Detached head: can't reset"]

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>