Support the Sapling identity in aliases

[bolinfest]

Using the Mercurial docs, I was able to define a sync alias as follows:

[alias]
sync=!$HG pull && $HG rebase -r "draft()" -d master

Based on this:

sapling/eden/scm/sapling/commands/debugrunshell.py

Line 26 in 78bd513

env["HG"] = util.hgexecutable()

I believe only $HG is supported, though ideally something that reflects the identity would be used instead:

sapling/eden/scm/lib/identity/src/lib.rs

Lines 367 to 381 in 78bd513

	user: &UserIdentity {
	cli_name: "sl",
	product_name: "Sapling",
	long_product_name: "Sapling SCM",
	env_prefix: "SL_",
	config_user_directory: Some("sapling"),
	config_user_files: &["sapling.conf"],
	#[cfg(windows)]
	config_system_path: r"Sapling\system.conf",
	#[cfg(not(windows))]
	config_system_path: "/etc/sapling/system.conf",
	scripting_env_var: "SL_AUTOMATION",
	scripting_config_env_var: "SL_CONFIG_PATH",
	scripting_except_env_var: "SL_AUTOMATION_EXCEPT",
	},

It would also be good to document this on the Sapling website so users don't have to make guesses based on the old Mercurial docs.