hg-fast-export.sh: let git manage the marks

hg-fast-export.sh: let git manage the marks

Ever since Git 1.5.1, "git fast-import" has supported an "--import-marks"
command and allowed the same file name to be used without conflict for both
the "--import-marks" and "--export-marks" options.

Therefore get rid of the kludgey code that exports to a temp file and then
attempts to combine the old and the new with uniq etc. etc.

By allowing Git to handle this the code is much simpler, Git always knows
about all the marks and there's no concern about the input to the 'uniq'
command not being in sorted order.

However, do export the marks to a different file so that in the unlikely
event of a disk full error where "git fast-import" fails to write the new
"--export-marks" file, no marks are lost and on success just replace the
old marks file with the new one.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>