cmdutil: separate building commit text from 'commitforceeditor'

This separation makes it easier to extend/hook building commit text
from the specified context.

This patch uses 'committext' instead of 'edittext' for names of newly
added variable and function, because the former is more purpose
specific than the latter, even though 'edittext' in 'buildcommittext'
is left as it is to reduce amount of diff.

mercurial/cmdutil.py

@@ -2170,6 +2170,23 b' def commiteditor(repo, ctx, subs):'
     return commitforceeditor(repo, ctx, subs)
 
 def commitforceeditor(repo, ctx, subs, finishdesc=None, extramsg=None):
+    committext = buildcommittext(repo, ctx, subs, extramsg)
+
+    # run editor in the repository root
+    olddir = os.getcwd()
+    os.chdir(repo.root)
+    text = repo.ui.edit(committext, ctx.user(), ctx.extra())
+    text = re.sub("(?m)^HG:.*(\n|$)", "", text)
+    os.chdir(olddir)
+
+    if finishdesc:
+        text = finishdesc(text)
+    if not text.strip():
+        raise util.Abort(_("empty commit message"))
+
+    return text
+
+def buildcommittext(repo, ctx, subs, extramsg):
     edittext = []
     modified, added, removed = ctx.modified(), ctx.added(), ctx.removed()
     if ctx.description():
@@ -2197,19 +2214,8 b' def commitforceeditor(repo, ctx, subs, f'
     if not added and not modified and not removed:
         edittext.append(_("HG: no files changed"))
     edittext.append("")
-    # run editor in the repository root
-    olddir = os.getcwd()
-    os.chdir(repo.root)
-    text = repo.ui.edit("\n".join(edittext), ctx.user(), ctx.extra())
-    text = re.sub("(?m)^HG:.*(\n|$)", "", text)
-    os.chdir(olddir)
 
-    if finishdesc:
-        text = finishdesc(text)
-    if not text.strip():
-        raise util.Abort(_("empty commit message"))
-
-    return text
+    return "\n".join(edittext)
 
 def commitstatus(repo, node, branch, bheads=None, opts={}):
     ctx = repo[node]