simplemerge: add optional context parameters to simplemerge

Rename the existing parameters for clarity.

These will, in subsequent patches, allow callers to redirect reads (of the
three sides of the merge) and writes (of the result) to the given contexts,
instead of using the filesystem.

While in most cases, the writes will go to a workingfilectx, this opens the
door for it to be a memfilectx in the case of an in-memory merge.

Repo will be necessary in a subsequent comit.

Differential Revision: https://phab.mercurial-scm.org/D372

mercurial/simplemerge.py

@@ -408,7 +408,8 b' def _verifytext(text, path, ui, opts):'
             raise error.Abort(msg)
     return text
 
-def simplemerge(ui, local, base, other, **opts):
+def simplemerge(ui, localfile, basefile, otherfile,
+                localctx=None, basectx=None, otherctx=None, repo=None, **opts):
     def readfile(filename):
         f = open(filename, "rb")
         text = f.read()
@@ -421,8 +422,8 b' def simplemerge(ui, local, base, other, '
         name_b = None
         name_base = None
     else:
-        name_a = local
-        name_b = other
+        name_a = localfile
+        name_b = otherfile
         name_base = None
         labels = opts.get('label', [])
         if len(labels) > 0:
@@ -435,16 +436,16 b' def simplemerge(ui, local, base, other, '
             raise error.Abort(_("can only specify three labels."))
 
     try:
-        localtext = readfile(local)
-        basetext = readfile(base)
-        othertext = readfile(other)
+        localtext = readfile(localfile)
+        basetext = readfile(basefile)
+        othertext = readfile(otherfile)
     except error.Abort:
         return 1
 
-    local = os.path.realpath(local)
+    localfile = os.path.realpath(localfile)
     if not opts.get('print'):
-        opener = vfsmod.vfs(os.path.dirname(local))
-        out = opener(os.path.basename(local), "w", atomictemp=True)
+        opener = vfsmod.vfs(os.path.dirname(localfile))
+        out = opener(os.path.basename(localfile), "w", atomictemp=True)
     else:
         out = ui.fout