Allow explicit setting of revision string

This lets a packager ensure that the binary object built from an
unpacked source tarball is identical to the binary object created from
a live git tree.

I hope to use it in debian with something like -Drevno=debian/3.36.1-1
during the meson configuration, so that the help dialog box and the
bug reporting logs can identify the specific build.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

meson.build

@@ -129,7 +129,10 @@ libmessagingmenu_dep = dependency('messaging-menu', version: '>= 12.10', require
 # Build variables
 geary_id = 'org.gnome.Geary@0@'.format(profile)
 geary_version = meson.project_version()
-revno = run_command('build-aux/git_version.py').stdout().strip()
+revno = get_option('revno')
+if revno == ''
+  revno = run_command('build-aux/git_version.py').stdout().strip()
+endif
 
 gnome = import('gnome')
 i18n = import('i18n')

meson_options.txt

@@ -58,3 +58,8 @@ option(
   value: false,
   description: 'Whether to use explicit reference tracking.'
 )
+option(
+  'revno',
+  type: 'string',
+  description: 'Custom revision string (default extracted from "git describe")'
+)