phabricator: refactor `phabread` to write all patches at once

This will be necessary to create a first class `phabimport` command.  That
command requires a transaction, and will import all named patches within a
single transaction.  But if Phabricator queries also happen within the
transaction, that leaves open the chance that an exception is raised, the
transaction is abandoned, and the next command that is run will complain about
needing to run `hg recover`.

Differential Revision: https://phab.mercurial-scm.org/D8135

hgext/phabricator.py

@@ -1615,13 +1615,17 b' def getdiffmeta(diff):'
 def readpatch(ui, drevs, write):
     """generate plain-text patch readable by 'hg import'
 
-    write is usually ui.write. drevs is what "querydrev" returns, results of
+    write takes a list of (DREV, bytes), where DREV is the differential number
+    (as bytes, without the "D" prefix) and the bytes are the text of a patch
+    to be imported. drevs is what "querydrev" returns, results of
     "differential.query".
     """
     # Prefetch hg:meta property for all diffs
     diffids = sorted(set(max(int(v) for v in drev[b'diffs']) for drev in drevs))
     diffs = callconduit(ui, b'differential.querydiffs', {b'ids': diffids})
 
+    patches = []
+
     # Generate patch for each drev
     for drev in drevs:
         ui.note(_(b'reading D%s\n') % drev[b'id'])
@@ -1640,7 +1644,10 b' def readpatch(ui, drevs, write):'
                 header += b'# %s %s\n' % (_metanamemap[k], meta[k])
 
         content = b'%s%s\n%s' % (header, desc, body)
-        write(content)
+        patches.append((drev[b'id'], content))
+
+    # Write patches to the supplied callback
+    write(patches)
 
 
 @vcrcommand(
@@ -1672,7 +1679,12 b' def phabread(ui, repo, spec, **opts):'
     if opts.get(b'stack'):
         spec = b':(%s)' % spec
     drevs = querydrev(repo.ui, spec)
-    readpatch(repo.ui, drevs, ui.write)
+
+    def _write(patches):
+        for drev, content in patches:
+            ui.write(content)
+
+    readpatch(repo.ui, drevs, _write)
 
 
 @vcrcommand(