Improve performances on high latency connexion

[Compostella, Jeremy]

Hi,

Sometime I'm working from home with a good Internet connexion.  Still,
going throught the VPN generates huge latencies to access the different
machines I'm working on.

Running a grep over tramp command takes more than 30 seconds so I
investigated a little bit today.  I found out that file properties has a
cache that I could use by customizing the
`remote-file-name-inhibit-cache' variable.  Its makes me save a bit more
than 10 seconds (according to ELP).

I had still to pay more than 20 seconds to get a grep result while in
the meantime from the console it takes less than 2 seconds ... I can
afford to wait a little more than using a console because it's more
convenient using Emacs but not that much.

I investigated a little bit more and figured out that a lot of shell
commands are sent especially to set environment variables.  I've
gathered these set variable commands in two commands and according to
ELP I've saved another 12 seconds.  Now running grep from Emacs over
tramp gets me result in less than 10 seconds which become acceptable.

I've joined the patch I've made, could you please review it (merge it
?).

I have a question too: is there a way to use a "persistent" tramp
connexion to run commands ?

Thanks,

Jérémy

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0001-tramp.sh.el-tramp-open-connection-setup-interactive-.patch
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