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

@@ -14,6 +14,8 b''
 # 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
@@ -51,6 +53,61 b' def normalize_po_files(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):