filemerge: call premerge directly from main merge function

The merge code currently does (in pseudocode):

for f in tomerge:
    premerge f
    merge f

We'd like to change this to look more like:

for f in tomerge:
    premerge f

for f in tomerge:
    merge f

This makes sure as many files are resolved as possible before prompting for the
others. This restructuring is also necessary for custom merge drivers.

This function separates out the premerge step from the merge step. In future
patches we'll actually turn these into separate steps in the merge driver.

The 'if r:' occurrences will be cleaned up in subsequent patches.

mercurial/filemerge.py

@@ -247,7 +247,7 b' def _merge(repo, mynode, orig, fcd, fco,'
     files. It will fail if there are any conflicts and leave markers in
     the partially merged file. Markers will have two sections, one for each side
     of merge, unless mode equals 'union' which suppresses the markers."""
-    r = _premerge(repo, toolconf, files, labels=labels)
+    r = 1
     if r:
         a, b, c, back = files
 
@@ -349,7 +349,7 b' def _idump(repo, mynode, orig, fcd, fco,'
     ``a.txt``, these files will accordingly be named ``a.txt.local``,
     ``a.txt.other`` and ``a.txt.base`` and they will be placed in the
     same directory as ``a.txt``."""
-    r = _premerge(repo, toolconf, files, labels=labels)
+    r = 1
     if r:
         a, b, c, back = files
 
@@ -361,7 +361,7 b' def _idump(repo, mynode, orig, fcd, fco,'
     return False, r
 
 def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
-    r = _premerge(repo, toolconf, files, labels=labels)
+    r = 1
     if r:
         tool, toolpath, binary, symlink = toolconf
         a, b, c, back = files
@@ -519,8 +519,15 b' def filemerge(repo, mynode, orig, fcd, f'
         if markerstyle != 'basic':
             labels = _formatlabels(repo, fcd, fco, fca, labels)
 
-        needcheck, r = func(repo, mynode, orig, fcd, fco, fca, toolconf, files,
-                            labels=labels)
+        r = 1
+        if mergetype == fullmerge:
+            r = _premerge(repo, toolconf, files, labels=labels)
+
+        if not r:  # premerge successfully merged the file
+            needcheck = False
+        else:
+            needcheck, r = func(repo, mynode, orig, fcd, fco, fca, toolconf,
+                                files, labels=labels)
 
         if not needcheck:
             if r: