cmdutil: omit redundant "savecommitmessage()" in "tryimportone()"
The preceding patch causes that "makememctx()" with "editor" argument
saves (manually edited) commit message into ".hg/last-message.txt":
saving itself is executed indirectly in "memctx.__init__()".
This makes it redundant to invoke "savecommitmessage()" on caller side
of "makememctx()".
This patch omits such redundant "savecommitmessage()" invocation in
"tryimportone()".
"tryimportone()" uses one of "commiteditor" or "commitforceeditor" as
"editor" argument, and this causes saving commit message always.
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