scripts/i18n: add command 'normalized-merge' for use with Mercurial's 'merge-tool' option

Add a 'normalized-merge' command to scripts/i18n that will first normalize
the i18n files contributing to the merge, then perform a standard merge. If
that merge fails (e.g. due to real conflicts) the normalized files are left
behind, and the user needs to run another merge tool manually and resolve the
merge of these.

Use by putting following snippets in your .hgrc file:

    [merge-tools]
    i18n.executable = /path/to/scripts/i18n
    i18n.args = normalized-merge $local $base $other $output

or

    [merge-tools]
    i18n.executable = python3
    i18n.args = /path/to/scripts/i18n normalized-merge $local $base $other $output

and when i18n files conflict, get all 3 sides of the merge normalized before
merge by running:

    hg resolve 'kallithea/i18n/*/LC_MESSAGES/kallithea.po' --tool i18n

scripts/i18n

@@ -1,61 +1,118 b''
 #!/usr/bin/env python3
 
 # -*- coding: utf-8 -*-
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
 # the Free Software Foundation, either version 3 of the License, or
 # (at your option) any later version.
 #
 # This program is distributed in the hope that it will be useful,
 # but WITHOUT ANY WARRANTY; without even the implied warranty of
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 # GNU General Public License for more details.
 #
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import os
+import shutil
 import sys
 
 import click
 
 import i18n_utils
 
 
 """
 Tool for maintenance of .po and .pot files
 
 Normally, the i18n-related files contain for each translatable string a
 reference to all the source code locations where this string is found. This
 meta data is useful for translators to assess how strings are used, but is not
 relevant for normal development nor for running Kallithea. Such meta data, or
 derived data like kallithea.pot, will inherently be outdated, and create
 unnecessary churn and repository growth, making it harder to spot actual and
 important changes.
 """
 
 @click.group()
 @click.option('--debug/--no-debug', default=False)
 def cli(debug):
     if (debug):
         i18n_utils.do_debug = True
     pass
 
 @cli.command()
 @click.argument('po_files', nargs=-1)
 def normalize_po_files(po_files):
     """Normalize the specified .po and .pot files.
 
     Only actual translations and essential headers will be preserved.
     """
     for po_file in po_files:
         i18n_utils._normalize_po_file(po_file, strip=True)
 
 @cli.command()
[e-mail]('local')
[e-mail]('base')
[e-mail]('other')
[e-mail]('output')
+def normalized_merge(local, base, other, output):
+    """Merge tool for use with 'hg merge/rebase/graft --tool'
+
+    Merging i18n files with a standard merge tool could yield merge conflicts
+    when one side is normalized and the other is not. In such cases, it may be
+    better to first normalize all sides, then proceed with a standard merge.
+    This command does exactly that, and can be used as 'merge-tool' in
+    Mercurial commands like merge, rebase and graft.
+
+    Add the following to your user or repository-specific .hgrc file to use it:
+        [merge-tools]
+        i18n.executable = /path/to/scripts/i18n
+        i18n.args = normalized-merge $local $base $other $output
+
+    and then invoke merge/rebase/graft with the additional argument '--tool i18n'.
+    """
+    from mercurial import (
+        context,
+        simplemerge,
+        ui as uimod,
+    )
+
+    print('i18n normalized-merge: merging file %s' % output)
+
+    i18n_utils._normalize_po_file(local, strip=True)
+    i18n_utils._normalize_po_file(base, strip=True)
+    i18n_utils._normalize_po_file(other, strip=True)
+    i18n_utils._normalize_po_file(output, strip=True)
+
+    # simplemerge will write markers to 'local' if it fails, keep a copy without markers
+    localkeep = local + '.keep'
+    shutil.copyfile(local, localkeep)
+
+    ret = simplemerge.simplemerge(uimod.ui.load(),
+         context.arbitraryfilectx(local.encode('utf-8')),
+         context.arbitraryfilectx(base.encode('utf-8')),
+         context.arbitraryfilectx(other.encode('utf-8'))
+    )
+    shutil.copyfile(local, output)  # simplemerge wrote to local
+    if ret:
+        basekeep = base + '.keep'
+        otherkeep = other + '.keep'
+        shutil.copyfile(base, basekeep)
+        shutil.copyfile(other, otherkeep)
+        sys.stderr.write("Error: simple merge failed. Run a merge tool manually to resolve conflicts, then use 'hg resolve -m'.\n")
+        sys.stderr.write('Resolve with e.g.: kdiff3 %s %s %s -o %s\n' % (basekeep, localkeep, otherkeep, output))
+        sys.exit(ret)
+
+    os.remove(localkeep)
+
[e-mail]()
 @click.argument('file1')
 @click.argument('file2')
 def normalized_diff(file1, file2):
     """Compare two files while transparently normalizing them."""
     sys.exit(i18n_utils._normalized_diff(file1, file2, strip=True))
 
 if __name__ == '__main__':
     cli()