add -i, send-email, svn, p4, etc: use "git var GIT_EDITOR"

Use the new "git var GIT_EDITOR" feature to decide what editor to
use, instead of duplicating its logic elsewhere.  This should make
the behavior of commands in edge cases (e.g., editor names with
spaces) a little more consistent.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -323,7 +323,7 @@ ENVIRONMENT AND CONFIGURATION VARIABLES
 The editor used to edit the commit log message will be chosen from the
 GIT_EDITOR environment variable, the core.editor configuration variable, the
 VISUAL environment variable, or the EDITOR environment variable (in that
-order).
+order).  See linkgit:git-var[1] for details.
 
 HOOKS
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