message_fetch: Fix home_msg_list newer FetchStatus tracking.
message_fetch: Fix home_msg_list newer FetchStatus tracking.

When we're doing the load_more frontfill, we were not correctly
declaring that we were in the process of doing a fetch.  Because the
next load_more call clears this state anyway, this was generally a
short race, off-screen, but it is still a data flow bug.

See the upcoming commits for a refactor that will eliminate the
possibility of this sort of bug.

shubhamdhama/zulip,tommyip/zulip,andersk/zulip,kou/zulip,showell/zulip,dhcrzf/zulip,timabbott/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,tommyip/zulip,kou/zulip,timabbott/zulip,synicalsyntax/zulip,punchagan/zulip,dhcrzf/zulip,timabbott/zulip,eeshangarg/zulip,hackerkid/zulip,brainwane/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,shubhamdhama/zulip,rishig/zulip,rht/zulip,shubhamdhama/zulip,kou/zulip,zulip/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,rishig/zulip,synicalsyntax/zulip,rht/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,tommyip/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,eeshangarg/zulip,dhcrzf/zulip,tommyip/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,dhcrzf/zulip,synicalsyntax/zulip,showell/zulip,tommyip/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,dhcrzf/zulip,hackerkid/zulip,brainwane/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,tommyip/zulip,andersk/zulip,showell/zulip,dhcrzf/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,dhcrzf/zulip,rishig/zulip,brainwane/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,kou/zulip,kou/zulip,andersk/zulip,rishig/zulip,tommyip/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,shubhamdhama/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,rishig/zulip,eeshangarg/zulip,rishig/zulip,showell/zulip,kou/zulip,rht/zulip